Method for treating molten metal

[0001] The invention relates to a method of treating molten metal in a metallurgical vessel, wherein a fine coating material comprising silicon oxide and alumina is applied to the surface of the molten metal.

[0002] It is known to coat molten metal surfaces in metallurgical vessels with a layer of high-melting silicon oxide covering material. This cover serves on the one hand to protect molten metal from atmospheric gases (especially oxygen). On the other hand, an effective thermal insulation is obtained through the coating, so that the cooling of the molten metal during processing is slowed down. Such methods are necessary especially in foundries or steel mills, because only then it is possible to effectively protect molten metal by pouring and placing a covering material on the surface of the molten metal.

[0003] It is well known to use various powdered coating materials, such as perlite or carbon black as waste material for the above purposes. In addition, it is known to use granular ash of vegetable origin, especially rice chaff ash, to which, for granulation, cellulose pulp, such as paper material or slurry, is added as an organic binder. As further added materials, graphite and slag are used in the previously known coating material. In addition, it may contain, among others, synthetic resins and colloidal silicic acid as additional binders.

[0004] Disadvantageously, for the previously known covering material based on vegetable ash, a relatively effortless production method is needed to obtain the physical properties needed for the desired purposes. In addition, a number of added materials are necessary, which generally leads to the fact that the previously known coating material is not taken into account for regular use in the steel industry due to its high costs.

[0005] A disadvantage of using conventional powder coating materials is their relatively low melting point of only about 1100 ° C to 1200 ° C. Due to the specific physical properties of the powders applied to the surface of the molten metal, they merge over time with a layer of slag also floating on the surface. As a result, on the one hand, the amount of slag is unfavorably strongly increased, and on the other hand, the constantly deteriorating heat insulation leads to the fact that the cooling slag hardens and settles on the vessels used for molten metal, which leads to cleaning works. The disadvantage is also the tendency of previously known covering materials to sinter on the surface of the molten metal and with the walls of the metallurgical vessels into a continuous layer. The low melting point of previously known covering materials also results in an adversely high consumption.

[0006] A particular problem with coating metal baths found in metallurgical vessels is that the materials used according to the state of the art often have unfavorable dust production. Due to the extreme convection of air above the hot molten metal, even the smallest amounts of dust are released in an uncontrolled manner into the environment.

[0007] It is disadvantageous when using previously known materials for coating molten metals is that they can only be spread on the surface of the molten metal in a bad way. In order to obtain an even surface coverage, it is often necessary to use circulation and thus expensive spreading devices.

[0008] In this context, the present invention is based on the task of developing a method for coating molten metal surfaces that avoids the above drawbacks. In particular, as little dust as possible can be ensured when applying the covering material to the surface of the molten metal. At the same time, the covering material used should have good thermal insulation properties and be able to spread easily on the surface of the molten metal.

[0009] This task is solved by the invention starting from the method of the type mentioned at the beginning by the fact that the coating material is an artificial zeolite material containing substantially equal proportions of silicon oxide and alumina, the coating material having a narrow grain diameter distribution, so that less than 20% have a diameter less than 30 μm or more than 125 μm.

[0010] Zeolites are, as is known, microporous crystals made of aluminum silicate networks. Due to the high porosity, these materials have excellent thermal insulation properties. The low density of these materials guarantees at the same time that the fine-grained coating material cannot sink and cannot combine with floating slag on molten metal, so that overgrowing of metallurgical vessels with sludge is effectively avoided. Due to the low specific weight of the artificial zeolite material, a certain separation of the covering layer from the liquid phase of the molten metal is ensured.

[0011] It is important in the method according to the invention that the fine-grained coating material has the narrow grain diameter distribution mentioned above. Due to the narrow grain diameter distribution, the powder coating material applied to the molten metal creates particularly large voids between the individual grains. Due to these voids, the thermal insulation properties are further improved. It is further particularly preferred that the coating material used according to the invention has excellent flowability on the surface of the molten metal. The covering material is thus arranged on the surface of the quasi-molten metal alone, so that any spreading device can be dispensed with. While the coating materials previously known in the art, after pouring onto the surface of the molten metal, remain there first lying in the form of a cone and then have to be spread using suitable devices, the coating material applied according to the invention to the molten metal melts on its own and covers it evenly molten metal surface. This extremely favorable behavior for the method according to the invention is based primarily on the narrow grain diameter distribution of the artificial zeolite material. The flowability of the covering material is further improved when its grains are substantially spherically shaped and have a surface as smooth as possible.

[0012] The covering material suitable in the sense of the invention is an artificial zeolite material, also referred to as so-called equilibrium-Catalyst, and produced in large quantities in petrochemicals as waste material. This zeolite material used in the production of gasoline from crude oil may, if appropriate after prior preparation, be made available at low cost for use in the process of the invention. It is particularly advantageous that large volumes of waste materials otherwise associated with their disposal are recycled to the appropriate application. In order to achieve the grain diameter distribution according to the invention, it may be necessary to mix and / or classify the used zeolite materials arising in the chemical industry with various grain diameter separations in a suitable manner.

[0013] Typical zeolite materials used in petrochemistry consist in substantially equal parts of alumina and silicon oxide. The remaining components, usually contained in these materials, do not interfere with the application of the molten metals according to the invention for coating, on the contrary they even have positive effects in the sense of the invention. If the ratio of alumina to silicon oxide is greater than one, the advantage arises that the coating material has a particularly high melting point of about 1500 ° C. In addition, silicon oxide cannot advantageously be used as an oxygen supplier for molten aluminum steels.

[0014] In practice, it has been found that the covering material used according to the invention can contain up to 1% of titanium oxide, iron oxide, magnesium oxide and calcium oxide in each case. Consequently, the essential properties of the covering material for the use according to the invention are not significantly impaired.

[0015] The method according to the invention is particularly suitable for coating molten metal present in a steel distributor or in a steel casting ladle. In a steel manifold (also called "tundish") or in a steel casting ladle, molten steel remains for a relatively long period of time, so that effective thermal insulation is important. In addition, molten steel has a large surface area in these vessels that needs to be covered. For this reason, the zeolite material used according to the invention is particularly suitable for its excellent flowability.

[0016] A suitable further embodiment of the method according to the invention is that the coating powder is applied to an intermediate layer of reactive calcium and aluminate slag located directly on the molten metal. In this two-layer coating system, the reactive calcium and aluminate slag serves to effectively protect molten metal from atmospheric gases on the one hand. On the other hand, the reactive calcium and aluminate slag advantageously results in the ingestion of unwanted non-metallic molten metal inclusions into the cover layer.

[0017] Artificial zeolite material applied to the reactive slag provides effective thermal insulation.

[0018] If desired, a carbon supplier, such as petroleum coke, may be added to the coating material applied according to the invention to the molten metal to affect the melting properties of the coating material. As a result, petroleum coke can also be sensibly used as waste material, also in petrochemicals.

[0019] In practice, the covering material can be applied to the surface of the molten metal particularly easily packaged in portions into plastic polymer bags. Due to the high temperatures above the molten metal clearly above 1000 ° C, plastic polymer bags immediately burn. Then the covering material is spread independently on the surface of the molten metal due to its flowability. Any generation of dust is effectively avoided, which is due to the fact that the grains of the covering material used according to the invention have a grain diameter of at least 30 μm.

[0020] According to a sensible further embodiment of the method according to the invention, the covering material consists of up to 40 wt. from vegetable ash. The rice chaff ash mentioned at the outset is particularly suitable. The addition of vegetable ash has the advantage that the insulation properties of the coating powder are further improved. In addition, vegetable ash is a coal supplier, which positively affects the melting properties of slag.

[0021] An important disadvantage of using pure plant ash is that the ash changes at high temperatures into the fibrous crystalline phase (kristobalite). These crystal fibers penetrate the lungs and can cause cancer. This disadvantage is effectively avoided by the plant ash mixture with artificial zeolite material according to the invention. The formation of kristobalite in the mixture is strongly inhibited.

In practice, for coating molten steels it has proved to be particularly effective to use a blend with at least 60 wt. zeolite material with up to 40 wt. plant ash. A blend of about 80 wt.% Is preferred. zeolite material with about 20 wt. ash chaff. This mixture preferably has almost neutral properties, while traditional coating materials usually react acidally and damage the linings of metallurgical vessels for a long time.

[0023] It is important that with the addition of vegetable ash within the previously given limits, the excellent flow properties of the coating material remain only preserved if the grain diameter distribution of the zeolite material according to the invention is observed.

[0024] An embodiment of the invention will be discussed below.

[0025] The attached drawing shows a grain diameter distribution diagram of an artificial zeolite material suitable according to the invention as a covering material. The chart shows the cumulative distribution function (v

%) relative to grain diameter (in μm). From the graph, it can be seen that about 80% of the grains have a diameter between 35 and 125 μm. The narrow grain diameter distribution resulting from the graph conditions the advantageous properties of the covering material used according to the invention described above.

[0026] The following table shows the diameter distribution of the zeolite material by numbers:

<td>&lt;32 μm</td><td> 7,8 %</td>

<td>&gt; 32 μm</td><td> 92,2 %</td>

<td>&gt; 45 μm</td><td> 79,2 %</td>

<td>&gt; 63 μm</td><td> 54,3 %</td>

<td>&gt; 90 μm</td><td> 22,5 %</td>

<td>&gt; 125 μm</td><td> 4,9 %</td>

<td>&gt; 250 μm</td><td> 0,0 %</td>

[0027] In terms of chemical composition, the covering material with the above grain diameter distribution consists of approximately 45% of silicon oxide and slightly more of alumina. Specific weight of the material₃ between 0.8 and 0.9 g / cm³.