DE102020127374A1 - Process for decomposing a metal carbonate and/or metal hydroxide to their metal oxide by means of microwaves - Google Patents

Abstract

The invention relates to a method for decomposing a metal carbonate and/or metal hydroxide to its metal oxide, comprising: a) mixing the metal carbonate and/or metal hydroxide with a microwave-absorbing material, andb) irradiation with microwaves, and the corresponding use of a microwave generator or a microwave-absorbing material.

Description

The invention relates to the extraction of metal oxides from ores, minerals and sedimentary rocks, or also from purified chemical substances, in particular the decomposition of a metal carbonate and/or metal hydroxide to their metal oxide by means of microwaves.

Among other things, oxides are obtained from the corresponding carbonates or hydroxides. The process is called calcination and is a decomposition.

DE102010050495A1 describes the calcination of aluminum hydroxide in a fluidized bed reactor from a primarily energetic point of view. It aims at preheating a filter with washing water.

In other known methods of decomposing metal carbonates or metal hydroxides, the heat is generated by combustion. If the combustion takes place directly in the reaction space of the decomposition, the disadvantage is that the product mixture is contaminated with combustion gases and combustion residues.

In the so-called Calix process, on the other hand, a double tube is used (“tube in a tube”), with combustion taking place in the outer tube to generate the necessary heat, and the desired decomposition taking place in the inner tube. The heat of combustion is transferred to the inner tube via the tube wall. The disadvantage here is that the pipe wall (e.g. made of steel) is heavily stressed by the temperature and that the energy transfer is inefficient. Overall, more must be burned in the outer tube than the energy required for decomposition in the inner tube.

Processes that do not require combustion usually work with roasting ovens, i.e. electric ovens.

US5,525,783 discloses a microwave system with a conveyor belt made of a microwave-reflecting material so that the microwaves reach the item on it from above. An embodiment of microwave-transparent material is also disclosed, in which the microwave generator sits below the conveyor belt. CO ₂ formed is discharged with an air countercurrent. The microwaves are passed on to the material to be irradiated via permanently installed heating rods.

DE10260743A1 describes a system with a fluidized bed reactor in which microwave radiation is fed in through a waveguide.

US4,967,486 also discloses a system consisting of a fluidized bed reactor with a microwave generator, the moisture in the samples causing the heating.

The object of the invention is to provide a method for the decomposition of ores, minerals or sedimentary rocks or the purified substances into the corresponding metal oxides. In particular, it should enable the decomposition or calcination of metal carbonates or metal hydroxides to form the corresponding oxides. It should be as simple as possible. In particular, it should require few steps. Of course, a reaction that is as complete as possible is desirable so that the products obtained are pure, ie both the metal oxide and the CO ₂ formed. The time required for decomposition should be as short as possible. It should be possible to dispense with solvents.

1. a) Mischen des Metall-Carbonats und/oder Metall-Hydroxids mit einem Mikrowellen-absorbierenden Material, und
2. b) Bestrahlung mit Mikrowellen.

The subject matter of the invention is a method for the decomposition of a metal carbonate and/or metal hydroxide to metal oxide with the steps:

1. a) mixing the metal carbonate and/or metal hydroxide with a microwave absorbing material, and
2. b) Irradiation with microwaves.

When the metal carbonate or metal hydroxide decomposes, carbon dioxide or water is released and the corresponding “metal oxide” is formed. "Metal oxide" thus refers to the metal carbonate and/or metal hydroxide used.

Decomposition of metal carbonate or metal hydroxide in the context of the invention is synonymous with calcination. It also encompasses using more than just one carbonate or hydroxide or a mixture of carbonate and/or hydroxides.

For the purposes of the invention, “metal carbonate” or “metal hydroxide” means that these are the main components (ie are contained to >50% in the material used). In the case of a material that contains both metal carbonate and metal hydroxide, the sum of both must be taken into account and both together must be >50%. It is preferably at least 70%, particularly preferably at least 80%. So it may be contaminated. This means that rocks, minerals or sediments are also used as “metal carbonate and/or metal hydroxide”, as long as they consist of >50% of the metal carbonate and/or metal hydroxide. These are, for example, minerals such as magnesite (MgCO ₃ ) or sedimentary rocks such as limestone (CaCO ₃ ).

Metal carbonates or hydroxides includes the respective metal and semimetal, alkali and also alkaline earth compounds, ie both heavy metal, light metal and noble metal compounds, as well as the carbonates or hydroxides of the base metals.

The metal carbonates also include the hydrogen carbonates (primary carbonates) and the dihydrogen carbonates (secondary carbonates).

“Microwave-absorbent material” within the meaning of the invention is a material that absorbs microwaves and converts them to a large extent into thermal energy, which is transferred to the metal carbonate and/or metal hydroxide by contact with it.

According to the invention, both b) and then a) take place first, i.e. the microwave-absorbing material is irradiated and then mixed with the metal carbonate and/or metal hydroxide in a) and the reverse order is included which is first mixed and then in b) the resulting mixture is irradiated. The third variant, in which a) and b) take place simultaneously, is also included.

So both are included: mixing and irradiation separately (in any order) or simultaneously.

According to the invention, a good heat transfer between the microwave-absorbing material and the metal carbonate or metal hydroxide takes place with the invention. The mixture according to the invention after or during the irradiation is very homogeneous with a large contact surface between microwave-absorbing material and metal carbonate or metal hydroxide.

The invention also relates to the use of a microwave generator for the decomposition of mixtures of a metal carbonate and/or metal hydroxide with a microwave-absorbing material to metal oxide; in particular the use in the method according to the invention.

In addition, the subject is also the use of a microwave-absorbing material for the decomposition of a metal carbonate and/or metal hydroxide to metal oxide by means of microwaves; Finally, in particular in the process according to the invention, the subject of the invention is also the use of a plant for the decomposition of a metal carbonate and/or metal hydroxide to metal oxide, comprising a microwave generator and a plant part selected from a fluidized bed reactor; Rotary kiln and shaft furnace, particularly preferably fluidized bed reactor, especially in the process according to the invention.

The invention advantageously provides pure metal oxides from the metal carbonates or the metal hydroxides.

The gases obtained, such as CO ₂ or H ₂ O vapor, are very pure and hardly or not at all contaminated with other gases, in particular not with combustion gases.

The advantage is that the microwave-absorbing material can be easily separated from the metal oxide and at the same time the heating takes place locally and does not just have to be conducted from the outside/the reaction vessel into the inside.

The procedure has few steps and is simple.

Due to the effective heating of the metal carbonate and/or metal hydroxide, the process is fast and the invention enables a time saving. Solvents are advantageously avoided.

Another advantage of the invention that is important for further use of the CO ₂ (for example for methanol synthesis) is that the process works in the presence of carbon, which causes the Boudouard equilibrium (between CO ₂ and CO over glowing C). The input of heat can be controlled very precisely by the quantity of microwave-absorbing material added in combination with the selected radiation energy. It is an advantage of the invention that the contact of the metal carbonate or metal hydroxide with the microwave-absorbing material is very good. Very good and homogeneous mixing is therefore advantageous.

The pure CO/CO ₂ or pure CO ₂ gas can be used in a variety of ways, eg in fuel synthesis (methanol), industry (eg the food industry), or storage without further conditioning effort.

In a preferred embodiment of the invention the mixing in a) takes place in a rotary kiln or fluidized bed mixer (i.e. a fluidized bed reactor).

The irradiation in b) preferably takes place in a fluidized bed reactor, a rotary kiln or a shaft furnace.

In a likewise preferred embodiment, the irradiation in step b) takes place in a fluidized bed reactor or a rotary tube, in particular in a fluidized bed reactor. The microwaver generator can also be outside of the interior. For the purposes of the invention, the impingement of the microwaves takes place inside the fluidized bed reactor or rotary tube. The good contact between the material to be decomposed (ie the metal carbonate and/or the metal hydroxide) and the microwave-absorbing material during the irradiation is particularly advantageous. The mixing that continues to take place during the irradiation also promotes the uniform release of the microwaves absorbed by the microwave-absorbing material to the material to be decomposed in the form of heat.

In a preferred embodiment of the invention, the mixing in a) and the irradiation in b) take place simultaneously.

Mixing (a) and irradiation (b) are particularly preferably carried out in the same reactor. A fluidized bed reactor or a rotary tube, which are expediently equipped with a microwave generator, are particularly suitable for this purpose. It is particularly preferably a fluidized bed reactor. However, this variant, in which both a) and b) take place in the same fluidized bed reactor or rotary kiln, also includes the use of a ready-made mixture of material to be decomposed and microwave-absorbing material.

Advantageously, particularly little microwave-absorbing material is required.

In a preferred embodiment of the invention, the temperature of the material to be irradiated during the irradiation in step b) is 500-1100°C. The temperature used during the irradiation depends on the decomposition temperature of the metal carbonate or hydroxide used in the process.

In a preferred embodiment of the invention, metal carbonates or metal hydroxides are used which have no or only a small dipole moment and therefore themselves absorb no or not appreciable microwaves. Such compounds are in particular magnesium carbonate, calcium carbonate and aluminum hydroxide.

In a preferred embodiment of the invention, the metal carbonate and/or metal hydroxide is independently selected from alkaline earth metal and alkali metal carbonates or hydroxides.

In the invention, the metal carbonate is preferably selected from magnesium carbonate and calcium carbonate and/or the metal hydroxide is selected from aluminum hydroxide and magnesium hydroxide.

In a preferred embodiment of the invention, the metal hydroxide is selected from Al(OH) ₃ and Mg(OH) ₂ .

In one embodiment with Al(OH) ₃ as the metal hydroxide, the temperature of the material to be irradiated is 900-1100° C., preferably 950±20° C., during the irradiation in step b).

If MgCO ₃ is decomposed with the invention, this temperature is preferably 500-700° C., particularly preferably 550-650° C., particularly 600±20° C., with CaCO ₃ at 800-1100° C., particularly preferably 900-1000° C, especially 950±20°C.

The temperature can also be significantly higher (up to 1800°C) if, for example, low porosity is desired in the products obtained, as is the case with the MgO product "dead burnt magnesia".

In a preferred embodiment of the invention, the microwave-absorbing material is selected from carbon (such as coke, petroleum coke or activated carbon), iron oxide, copper oxide and SiC, particularly preferably from carbon, in particular carbon.

In a likewise preferred embodiment, carbon or a metal oxide is used as the microwave-absorbing material.

In the variant of the invention in which carbon (as microwave-absorbing material) and metal carbonate (as material to be decomposed) are used, the well-known Boudouard equilibrium (CO ₂ +C ⇔ 2 CO) is established for the resulting CO ₂ over glowing carbon one. That is, the resulting CO ₂ gas would be contaminated with CO. In this variant, the temperature during the irradiation in b) is preferably at most 50° C. above the decomposition temperature of the respective metal carbonate. As is known, the Boudouard equilibrium is relatively far on the CO ₂ side at approx. 700°C and relatively far on the CO side at 1300°C. However, the kinetics of the Boudouard reaction are relatively slow at temperatures < 1200°K (923°C).

It is particularly advantageous that, with the invention, the temperature in the gas space of the irradiation in b) is significantly lower than on the surface of the microwave-absorbing material.

1. 1. Metall-Oxid + CO = Metall + CO₂
2. 2. Metall + 0.5 O₂ (Luft) = Metall-Oxid.

If CO is produced, but clean CO ₂ is desired, the CO will preferably also become CO ₂ converted via the following two partial reactions, for example via a "chemical looping" filter:

1. 1. metal oxide + CO = metal + CO ₂
2. 2. metal + 0.5 O ₂ (air) = metal oxide.

In a preferred embodiment of the invention, the microwave absorbing material additionally comprises an outer coating of the metal oxide produced by the method. In the example, this means that CaCO ₃ is to be decomposed, that, for example, a core made of SiC is coated with CaO.

The advantage of this embodiment is that abrasion during mixing does not lead to contamination of the product since only the desired product is rubbed off.

In a preferred embodiment of the invention, the metal carbonate and/or the metal hydroxide has a particle size in the range from 1 μm to less than 1 mm.

In one embodiment, the microwave-absorbing material is larger, preferably >1mm, in particular 1-2mm. when both versions are combined, the microwave-absorbing material can advantageously be easily separated again after the decomposition according to the invention, from the metal oxide obtained.

After the invention has been carried out, the microwave-absorbing material is preferably removed by means of sieving, air classification, or in a magnetic manner if one of the materials is magnetic.

An example of a magnetic microwave absorbing material is magnetite.

In a sieving, separation is based on the different particle sizes, for example as in the aforementioned embodiment. The screening can take place hot or after the mixture to be screened has been cooled.

With wind stratification, it is possible to separate the finer fraction (metal oxide) with the gas flow. Wind stratification can take place hot or after cooling the mixture to be screened.

With these variants, microwave-absorbing material that has been separated off again can be reused.

• 1 zeigt eine bevorzugte Ausführungsform der Erfindung, bei der die Zersetzung mittels Mikrowellen im Innenraum eines Wirbelschichtreaktors stattfindet.
• 2 zeigt eine bevorzugte Ausführungsform der Erfindung, bei der das Mikrowellen-absorbierende Material zusätzlich eine äußere Beschichtung aus dem Metall-Oxid umfasst, das mit dem Verfahren hergestellt wird.

In a preferred embodiment of the uses according to the invention, the decomposition takes place in the interior of a fluidized bed reactor, rotary tube reactor or shaft furnace, preferably a fluidized bed reactor or rotary tube reactor (=rotary tube), in particular a fluidized bed reactor. This means that it is used for decomposition in the interior of such a reactor or furnace.

• 1 shows a preferred embodiment of the invention, in which the decomposition takes place by means of microwaves in the interior of a fluidized bed reactor.
• 2 Figure 1 shows a preferred embodiment of the invention in which the microwave absorbing material additionally comprises an outer coating of the metal oxide produced by the method.

All of the embodiments mentioned can be combined with one another in any desired manner.

The invention is described by the following exemplary embodiments without being restricted to them.

exemplary embodiments

Example 1: CaCO ₃ / petroleum coke / rotary kiln / surface irradiation

CaCO ₃ is used and mixed in a rotary kiln with dry and previously degassed petroleum coke (ratio CaCO ₃ /petroleum coke: 1:0.5) and preheated at 200° C. for 10 minutes. The mixture is then spread over a large area and irradiated with microwaves over a large area. None of the CaCO ₃ particles (and thus also of the producing CaO) particles is larger than 800 µm. The pre-degassed petroleum coke particles are larger than 1.2 mm.

The radiation power of the microwave generator is selected in such a way that the temperature measurement in the flat mixture gives a value of approx. 900°C. The energy required to heat the mixture and to split CaCO ₃ is around 0.57 kWh/kg mixture. Finally, the petroleum coke residue is separated from the CaO obtained. The particle sizes mentioned above would allow this via sieving.

Exemplary embodiment 2: MgCO ₃ / biocoke / fluidized bed reactor

In the exemplary embodiment, MgCO ₃ (no particles larger than 0.7 mm) is mixed with biocoke (obtained by pyrolysis of biomass waste) in a fluidized bed reactor. The ratio between MgCO ₃ and biocoke is (1: 0.25). The irradiation takes place inside the fluidized bed reactor, so that an internal temperature of 600°C is reached. The energy required to heat the mixture and to realize the calcination of MgCO ₃ is about 0.45 kWh/kg mixture if the input material is not preheated. The biocoke (particles > 1mm) is then separated from the MgO by sieving.

Example 3: Al(OH) ₃ / copper oxide coated with Al ₂ O ₃ / fluidized bed reactor

Exemplary embodiment 3 is carried out analogously to exemplary embodiment 2. The ratio between Al(OH) ₃ and copper oxide as a microwave absorbing material is (1:0.25). However, Al(OH) ₃ with a particle size of 100 µm is used and mixed with copper oxide coated with Al ₂ O ₃ (particle size of the coated copper oxide total 1.5 mm). In the example, the irradiation takes place at 950°C. The energy required to heat the mixture and to realize the calcination of Al(OH) ₃ is about 0.5 kWh/kg mixture when the mixture is preheated at 250°C. After the irradiation has taken place, the copper oxide particle is removed by means of wind stratification.

Reference List

1

fluidized bed reactor

2

microwaves

3

Microwave absorbing material

4

metal carbonate and/or metal hydroxide

5

Metal Oxide Coating

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102010050495 A1 [0003]
• US5525783 [0007]
• DE 10260743 A1 [0008]
• US4967486 [0009]

Claims (10)

A method of decomposing a metal carbonate and/or metal hydroxide (4) to metal oxide comprising: a) mixing the metal carbonate and/or metal hydroxide (4) with a microwave absorbing material (3), and b) Irradiation with microwaves. procedure after claim 1 , where a) and b) take place simultaneously. procedure after claim 1 or 2 , where a) and b) take place in a fluidized bed reactor or in a rotary tube. Procedure according to one of Claims 1 until 3 , wherein the microwave-absorbing material (3) additionally comprises an outer coating of the metal oxide (5) produced by the method. Procedure according to one of Claims 1 until 4 , wherein carbon or a metal oxide is used as the microwave-absorbing material (3). Procedure according to one of Claims 1 until 5 wherein the metal carbonate and/or metal hydroxide (4) is independently selected from alkaline earth metal and alkali metal carbonates or hydroxides. Procedure according to one of Claims 1 until 6 , wherein the metal carbonate is selected from magnesium carbonate and calcium carbonate and/or the metal hydroxide is selected from aluminum hydroxide and magnesium hydroxide. Use of a microwave generator for the decomposition of mixtures of a metal carbonate and/or metal hydroxide (4) with a microwave-absorbing material (3) to metal oxide. Use of a microwave-absorbing material (3) for the decomposition of a metal carbonate and/or metal hydroxide (4) to metal oxide by means of microwaves (2). Use after one of Claims 8 or 9 , wherein the decomposition takes place in the interior of a fluidized bed reactor (1), rotary tube reactor or shaft furnace.

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