JPS5855815B2 - Manufacturing method of highly active denitrification catalyst - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a highly active denitrification catalyst.

The denitrification catalyst used in the so-called selective reduction flue gas denitrification method is a catalyst that coats the surface of steel with an aluminum layer.
It is known that steel and aluminum are then formed into a thermal diffusion alloy, and the aluminum is further eluted to activate the steel surface by oxidation (see Japanese Patent Laid-Open No. 52-4491).

This catalyst has a porous surface and exhibits fairly high activity, but it is still not perfect.

Therefore, to obtain higher activity, a larger surface area is required.

By the way, for steel materials whose surfaces are coated with an aluminum film, the thermal expansion coefficients of the base steel material and the steel-aluminum alloy layer are different, so tensile residual stress occurs as it cools, and in some cases, cracks may occur in the steel material. There is.

This invention was made with attention to the above-mentioned phenomenon, and the object is to increase the surface area of a catalyst by actively generating cracks, thereby obtaining a highly active catalyst.

The method for producing a highly active denitrification catalyst according to the present invention includes a step of coating the surface of a steel material with aluminum, a step of forming a thermal diffusion alloy between the steel material and aluminum, a step of eluting the aluminum to make the surface of the steel material porous, and a step of making the surface of the steel material porous. In a catalyst production method comprising a step of oxidizing the surface, after the coating step and/or after the thermal diffusion alloying step,
This method is characterized by rapidly cooling a high-temperature steel material having a solid coating layer or alloy layer.

The coating treatment temperature is the melting point of aluminum (660℃
) or higher, usually around 700°C.

Further, the thermal diffusion alloying treatment temperature is about 800°C.

In the rapid cooling process, a high temperature steel material having a solid coating layer or alloy layer is generally immersed in water.

The coefficient of thermal expansion of the steel material and the steel-aluminum alloy layer is
12 x 10-6/℃ and 17 x 10-6/℃ respectively
℃, cracks occur in the alloy layer due to the rapid cooling treatment.

In this case, the cracks enter in the thickness direction of the alloy layer, so that the alloy layer does not separate from the base of the steel material.

Therefore, when aluminum is subsequently eluted from the alloy layer, the eluting progresses rapidly and a porous body with a large surface area is obtained.

Therefore, if this is oxidized and activated, a denitrification catalyst with a thick active layer can be obtained.

The quenching treatment is performed after the coating step or after the thermal diffusion alloying step, and more preferably after both of these steps.

Note that when steel is immersed in a molten aluminum bath to be coated and then rapidly cooled, if water cooling is performed while molten aluminum is attached to the surface of the steel, the water will expand rapidly and cause an explosion. This is dangerous.

Therefore, water cooling is performed after the molten aluminum has solidified.

This is not only safe, but also allows you to remove excess fusic from the aluminum surface.

According to this invention, the high temperature steel material having a solid coating layer or alloy layer is rapidly cooled after the coating process and/or after the heat diffusion alloying process, so that cracks are generated in the alloy layer and the surface area is increased. Moreover, the active layer can be made thicker.

In this way, a highly active catalyst can be obtained, and as a result, the amount of catalyst used can be reduced, and the denitrification apparatus can be downsized.

Furthermore, the cooling period can be shortened, which is advantageous in terms of workability.

In addition, when slow cooling is performed as in the past, residual stress in the alloy layer is released in the form of peeling of the active layer because no cracks occur, but in this invention, residual stress is released due to cracks. Therefore, there is no risk of deterioration of the catalyst due to peeling of the active layer, etc. Example SUS 304 steel plate (10 mm x 50 mm x O, 5
mm) was immersed in a molten aluminum bath (approximately 700°C) for 5 minutes to coat the surface with aluminum.

Once the molten aluminum on the surface solidified, the steel plate was immediately cooled with water.

Next, the steel plate was heated to about 800° C. to undergo solid-phase diffusion alloying treatment between aluminum and steel, and then further water-cooled.

The above processing is shown in FIG. 1 along with conventional operations.

Next, the steel plate was immersed in an alkaline aqueous solution to dissolve the aluminum.

Hydrogen gas was generated at this time, and the amount of hydrogen gas generated is shown in FIG. 2 in comparison with the conventional method.

Finally, the steel surface was activated by oxidation. In this way, a denitrification catalyst having a thick active layer was obtained.

A cross section of this catalyst is shown in FIG. 3 in comparison with a conventional catalyst.

In FIG. 3, 1 is a steel base material, 2 is an alloy layer, and 3 is an active layer.

Next, the activity of this catalyst is compared with that of conventional catalysts and is shown in the table below.

[Brief explanation of the drawing]

FIG. 1 is a graph of the relationship between time and temperature showing an example of the present invention, FIG. 2 is a graph of the relationship between time and hydrogen generation amount, and FIG.
The figure is a cross-sectional view of the catalyst.

Claims (1)

[Claims] 1. A catalyst consisting of the steps of coating the steel surface with aluminum, forming a thermal diffusion alloy between the steel and aluminum, eluting aluminum to make the steel surface porous, and oxidizing the steel surface. A method for producing a highly active denitrification catalyst, which comprises subjecting a high-temperature steel material having a solid coating layer or alloy layer to a rapid cooling treatment after a coating step and/or a thermal diffusion alloying step.