Passivated pyrophoric metal, especially passivated magnesium, and process for its preparation

Abstract

The present invention relates to a process for passivating at least one pyrophoric metal, comprising the step of coating the at least one pyrophoric metal, such as preferably magnesium, with urea. Furthermore, the present invention relates to passivated pyrophoric metal, in particular magnesium, obtainable by the process in the form of pyrophoric metal particles coated with a urea-containing or urea-consisting coating, as well as to the use of passivated magnesium for producing pyrotechnics, coated welding electrodes, refractory products, flux-cored wires, or desulfurizing agents for pig iron desulfurization.

Description

[0001] The present invention relates to a process for passivating at least one pyrophoric metal, such as in particular magnesium, to passivated pyrophoric metal, in particular passivated magnesium, obtainable by the process, and to the use of passivated magnesium.

[0002] Metals have a wide range of uses, such as calcium as a reducing agent in metallurgy or as a reducing agent in steel and aluminum production, magnesium as a desulfurizer for pig iron or as an alloying component, and lead in the production of batteries or as an alloying component. However, a large number of metals, such as lead, calcium and magnesium, particularly when in the form of relatively small particles, are pyrophoric, i.e. spontaneously igniting, because they ignite spontaneously at low temperatures or even at room temperature as a result of reaction with the oxygen in the air, or are at least highly flammable. This limits the use of these metals and in particular their use in the form of small particles. For this reason, these metals are usually passivated beforehand, particularly when they are to be used in the form of small particles.Typically, passivation is achieved by coating the metal particles with a coating that reduces or even prevents their self-ignition and / or flammability. In the case of magnesium, passivation was primarily achieved with melamine for some time. However, melamine has recently been classified as a carcinogenic substance and is therefore no longer desirable as a passivating agent for pyrophoric metals.

[0003] Therefore, melamine must be replaced by other passivating agents, which are, however, less effective than melamine.

[0004] Against this background, the present invention is based on the object of specifying a method for passivating at least one pyrophoric metal, such as in particular magnesium, in which a passivating agent is used which is largely harmless to health, but which is as effective or at least almost as effective as melamine with regard to reducing the self-ignition or combustibility of the pyrophoric metal.

[0005] According to the invention, this object is achieved by a method for passivating at least one pyrophoric metal, which comprises the step of coating the at least one pyrophoric metal with urea.

[0006] This solution is based on the surprising discovery that urea, which is harmless to health, is an extremely effective passivating agent for pyrophoric metals, and especially for magnesium. Therefore, even comparatively small amounts of urea in the coating of the pyrophoric metal are sufficient to achieve effective passivation against spontaneous combustion or burning. Without wishing to be bound by any theory, it is believed that this is due to the fact that urea decomposes in endothermic reactions at temperatures just below 200°C, thereby extracting energy from the environment, so that in the event of a fire, burning metal cools down quickly and the metal fire is quickly extinguished. A further advantage of the process according to the invention is that the resulting passivated pyrophoric metal, unlike uncoated magnesium, is not hazardous material and can therefore be easily transported.

[0007] According to the invention, the pyrophoric metal is coated with urea. This includes coating exclusively with urea or with a mixture containing urea. For the purposes of the present invention, coating means that urea or a urea-containing mixture adheres to at least part of the pyrophoric metal, in particular to part of the particles of the pyrophoric metal, over the entire surface or in part. For example, more than 1 wt.%, in particular more than 5 wt.%, such as 1 to 100 wt.% or 5 to 60 wt.% or 10 to 40 wt.% of all metal particles are fully or partially coated with urea or a urea-containing mixture, whereas the remaining part of the particles is uncoated.In this context, "partially" means that urea or a urea-containing mixture adheres to a portion of the surface of a metal particle, such as 1 to 90%, in particular 5 to 70%, such as 10 to 60% of the surface of a metal particle. This adhesion can involve any type of chemical, physical, adhesive, or ionic bond, meaning the urea is bonded to the surface of the metal particle in some way. Typically, this bond is adhesive, with the urea sticking to the surface of the metal particle.

[0008] In principle, the present invention is not particularly limited with regard to the pyrophoric metal. Good results are obtained in particular when the at least one pyrophoric metal is selected from the group consisting of aluminum, lead, calcium, iron, magnesium, and any mixtures of two or more of the aforementioned metals. Particularly preferably, the at least one pyrophoric metal is magnesium.

[0009] As explained above, there is a risk of ignition or fire from pyrophoric metals, particularly when they are fine-grained, i.e., in the form of comparatively small particles, which is why passivation of such fine-grained metal particles in particular is advantageous. Therefore, in the process according to the invention, it is preferred to coat particulate pyrophoric metal, such as, in particular, magnesium, preferably with an average d 50 particle size of 0.01 to 10 mm and particularly preferably with an average d 50 particle size of 0.5 to 5 mm. For example, the pyrophoric metal, such as, in particular, magnesium, can be coated in the form of powder with an average d 50 particle size of 0.01 to 10 mm, in the form of granules with an average d 50 particle size of 0.01 to 10 mm, or in the form of chips with an average chip length of 0.01 to 10 mm.In the process according to the invention, pyrophoric metal, such as in particular magnesium, is particularly preferably coated in the form of powder with an average d50 particle size of 0.5 to 5 mm, in the form of granules with an average d50 particle size of 0.5 to 5 mm, or in the form of chips with an average chip length of 0.5 to 5 mm. Average particle size d50 is understood to mean the value for the particle diameter below which 50% of the particles present fall. The average particle size d50 can be determined in particular by laser diffractometry. In contrast, chip length refers to the longest extension of a chip along a straight line, and thus average chip length refers to the average value of all chip lengths.Particularly preferably, at least 80% and particularly preferably at least 90% of all powder or granulate particles have a particle size of 0.01 to 10 mm and preferably of 0.5 to 5 mm and at least 80% and particularly preferably at least 90% of all chips have a chip length of 0.01 to 10 mm and preferably of 0.5 to 5 mm.

[0010] In principle, the pyrophoric metal particles or metal chips can be coated with urea alone. Good results are obtained in this embodiment, especially when the coating is carried out by mixing the at least one pyrophoric metal with urea with an average d 50 particle size of 10 nm to 1 mm, and preferably 100 nm to 200 µm.

[0011] In order to improve the wettability of the at least one pyrophoric metal or the coatability of the at least one pyrophoric metal, it is proposed in a further development of the inventive concept to carry out the coating by mixing the at least one pyrophoric metal with a mixture of urea and at least one additive, wherein the at least one additive is selected from the group consisting of alkali metal oxides, alkali metal carbonates, alkali metal phosphates, alkali metal nitrates, alkali metal sulfates, alkaline earth metal oxides, alkaline earth metal carbonates, alkaline earth metal phosphates, alkaline earth metal nitrates, alkaline earth metal sulfates and any mixtures of two or more of the aforementioned compounds.

[0012] Particularly preferably, the at least one additive in this embodiment of the present invention is selected from the group consisting of alkali metal oxides, alkali metal carbonates, alkaline earth metal oxides, alkaline earth metal carbonates and any mixtures of two or more of the aforementioned compounds.

[0013] Good results are obtained in this embodiment of the present invention in particular when the at least one additive is selected from the group consisting of calcium oxide, magnesium oxide, calcium carbonate, magnesium carbonate and any mixtures of two or more of the aforementioned compounds.

[0014] According to a particularly preferred embodiment of the present invention, calcium oxide, magnesium oxide, calcium carbonate, or a mixture of calcium oxide and magnesium oxide is used as at least one additive, most preferably a mixture of calcium oxide and magnesium oxide. In the latter variant, the weight ratio of calcium oxide to magnesium oxide is preferably 10:1 to 1:1, more preferably 5:1 to 1:1.

[0015] In the above embodiment of the present invention, it is particularly preferred that both the urea and the at least one additive in the mixture used for coating each have an average d50 particle size of 10 nm to 1 mm and particularly preferably of 100 nm to 200 µm.

[0016] Good results are achieved in particular when the weight ratio of urea to the at least one additive in the mixture used to coat the at least one pyrophoric metal is 1:1 to 1:10.

[0017] According to a further preferred embodiment of the present invention, the step of coating the at least one pyrophoric metal with urea or with a mixture of urea and at least one additive is carried out at room temperature. Alternatively, the coating step can also be carried out at slightly elevated temperatures of 24 to 50°C or at slightly reduced temperatures of 1 to 22°C.

[0018] As explained above, in the process according to the invention even a small amount of urea in the coating is sufficient to effectively reduce the ignitability or combustibility of the at least one pyrophoric metal. Therefore, in a further development of the inventive concept, it is proposed to carry out the coating by mixing the at least one pyrophoric metal with urea or with a mixture of urea and at least one additive, wherein the urea or the mixture of urea and at least one additive is added to the pyrophoric metal in an amount of 0.1 to 10 wt.% based on 100 wt.% of the pyrophoric metal, so that preferably coated pyrophoric metal is obtained in which the amount of urea contained in the coating, based on 100 wt.% of the pyrophoric metal, is 0.1 to 10 wt.% and preferably 0.5 to 3 wt.%. Usually, not the entire amount of urea or urea used is used.Mixture of urea and at least one additive to bind to the pyrophoric metal.

[0019] According to a further aspect, the present invention relates to passivated pyrophoric metal in the form of pyrophoric metal particles coated with a urea-containing or urea-based coating. The particles may, in particular, be powder particles, granulate particles, chips, or the like.

[0020] The coating of the passivated pyrophoric metal preferably contains at least one additive selected from the group consisting of alkali metal oxides, alkali metal carbonates, alkali metal phosphates, alkali metal nitrates, alkali metal sulfates, alkaline earth metal oxides, alkaline earth metal carbonates, alkaline earth metal phosphates, alkaline earth metal nitrates, alkaline earth metal sulfates, and any mixtures of two or more of the aforementioned compounds. Preferably, the coating of the passivated pyrophoric metal contains, in addition to urea, calcium oxide, magnesium oxide, calcium carbonate, or a mixture of calcium oxide and magnesium oxide as an additive.

[0021] According to a further preferred embodiment of the present invention, the particles are magnesium particles with an average d50 particle size of 0.01 to 10 mm, and particularly preferably with an average d50 particle size of 0.5 to 5 mm. Particularly preferred are magnesium powder particles with an average d50 particle size of 0.01 to 10 mm, and preferably 0.5 to 5 mm, magnesium granulate particles with an average d50 particle size of 0.01 to 10 mm, and preferably 0.5 to 5 mm, or magnesium chips with an average chip length of 0.01 to 10 mm, and preferably 0.5 to 5 mm.

[0022] Good results are obtained in particular when the amount of urea contained in the coating, based on 100 wt% of the pyrophoric metal, is 0.1 to 10 wt% and preferably 0.5 to 3 wt%.

[0023] Finally, the present invention relates to the use of passivated magnesium as described above for producing pyrotechnics, coated welding electrodes, refractory products, flux-cored wires or desulfurizing agents for pig iron desulfurization.

Claims

1. A method for passivating at least one pyrophoric metal comprising the step of coating the at least one pyrophoric metal with urea.

2. The method of claim 1, wherein the at least one pyrophoric metal is selected from the group consisting of aluminum, lead, calcium, iron, magnesium, and any mixtures of two or more of the aforementioned metals.

3. The method of claim 1 or 2, wherein the at least one pyrophoric metal is magnesium.

4. A method according to any one of the preceding claims, wherein the at least one pyrophoric metal is present in particulate form and preferably in the form of powder with an average d 50 -Particle size of 0.01 to 10 mm and preferably 0.5 to 5 mm, in the form of granules with an average d 50-particle size of 0.01 to 10 mm and preferably 0.5 to 5 mm or in the form of chips with an average chip length of 0.01 to 10 mm and preferably 0.5 to 5 mm.

5. A method according to any one of the preceding claims, wherein the coating is carried out by mixing the at least one pyrophoric metal with urea having an average d 50 -Particle size of 10 nm to 1 mm and preferably from 100 nm to 200 µm.

6. The method according to any one of claims 1 to 4, wherein the coating is carried out by mixing the at least one pyrophoric metal with a mixture of urea and at least one additive, wherein the at least one additive is selected from the group consisting of alkali metal oxides, alkali metal carbonates, alkali metal phosphates, alkali metal nitrates, alkali metal sulfates, alkaline earth metal oxides, alkaline earth metal carbonates, alkaline earth metal phosphates, alkaline earth metal nitrates, alkaline earth metal sulfates and any mixtures of two or more of the aforementioned compounds.

7. The method of claim 6, wherein the at least one additive is calcium oxide, magnesium oxide, calcium carbonate or a mixture of calcium oxide and magnesium oxide.

8. The method according to claim 6 or 7, wherein the urea and the at least one additive in the mixture used for coating each have an average d 50-Particle size of 10 nm to 1 mm and preferably of 100 nm to 200 µm.

9. A method according to any one of the preceding claims, wherein the weight ratio of urea to the at least one additive in the mixture used to coat the at least one pyrophoric metal is 1:1 to 1:

10.

10. The method according to any one of the preceding claims, wherein the coating is carried out by mixing the at least one pyrophoric metal with urea or with a mixture of urea and at least one additive, wherein the urea or the mixture of urea and at least one additive is added to the pyrophoric metal in an amount of 0.1 to 10 wt.% based on 100 wt.% of the pyrophoric metal, so that preferably coated pyrophoric metal is obtained in which the amount of urea contained in the coating, based on 100 wt.% of the pyrophoric metal, is 0.1 to 10 wt.% and preferably 0.5 to 3 wt.%.

11. Passivated pyrophoric metal in the form of particles of pyrophoric metal coated with a coating containing or consisting of urea.

12. The passivated pyrophoric metal of claim 11, wherein the coating further contains calcium oxide, magnesium oxide, calcium carbonate, or a mixture of calcium oxide and magnesium oxide.

13. Passivated pyrophoric metal according to claim 11 or 12, wherein the particles are magnesium powder particles having an average d50 particle size of 0.01 to 10 mm and preferably 0.5 to 5 mm, magnesium granulate particles having an average d50 particle size of 0.01 to 10 mm and preferably 0.5 to 5 mm or magnesium chips having an average chip length of 0.01 to 10 mm and preferably 0.5 to 5 mm.

14. Passivated pyrophoric metal according to any one of claims 11 to 13, wherein the amount of urea contained in the coating, based on 100 wt.% of the pyrophoric metal, is 0.1 to 10 wt.% and preferably 0.5 to 3 wt.%.

15. Use of passivated magnesium according to any one of claims 11 to 14 for the manufacture of pyrotechnics, coated welding electrodes, refractory products, flux-cored wires or desulfurizing agents for pig iron desulfurization.

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