JPS5843353B2 - How to fire magnesia - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for firing magnesia having high density and sufficiently developed crystals.

Magnesia, like alumina, is one of the most widely used oxides, and is currently widely used as a material for basic steelmaking furnaces, as well as in fields that utilize its excellent properties, such as metal melting crucibles and optical applications. It is expected to be used as a refractory material for electronic materials, MHD power generation, etc.

It has been suggested that one way to improve the quality of magnesia refractories as refractories in steelmaking furnaces and to expand the uses of magnesia is to increase the density of magnesia sintered bodies and the size of their crystal grains. .

In other words, increasing the density of the sintered body can be expected to have effects such as suppressing slag corrosion, improving strength, and improving electrical properties, while enlarging crystal grains suppresses slag corrosion and improves creep characteristics at high temperatures. It can be expected to have effects such as improving

Hot pressing methods and methods of cutting magnesia from fused magnesia are known as methods for producing magnesia sintered bodies with high density and large crystal grains, but these methods are both expensive and difficult to obtain. It was thought that improvements to magnesia sintered bodies could only be made in very limited cases due to defects such as limited strength and a significant decrease in strength between walls.

Of course, various efforts have been made to increase the density of the sintered body in conventional magnesia sintering methods, such as molding light sintered magnesia and firing it in a rotary kiln, shaft kiln, etc.
For example, the addition of sintering accelerators, adjustment of impurity composition, and changes in the firing atmosphere have been taken.

However, despite these efforts, in the case of high-purity magnesia that has the characteristics of magnesia to a high degree, the bulk specific gravity is only 3.4 even if it is fired at a high temperature of 2000°C in the conventional method, and the crystal grain size is 30. ~40μ was considered to be the limit.

By improving the conventional magnesia firing method, the present invention not only makes it possible to fire magnesia with high density and large crystal grains that cannot be obtained with the conventional method, but also lowers the firing temperature to 2,000 yen as in the conventional method. They discovered a method that does not require high temperatures of ℃.

According to the method of the present invention, a magnesia sintered body having a bulk specific gravity of 3.5 or more and a crystal grain size of approximately 60 μm can be easily obtained at a sintering temperature of 1500° C. without using a complicated sintering method such as hot pressing.

In the present invention, a molded body of magnesium oxide is formed under a total pressure of 200 m
mHg or higher, contains 25 vol% or more of water vapor, and has a ratio of water vapor partial pressure to carbon dioxide gas partial pressure (H20/C02) of 1.
A method for firing magnesia, characterized by firing in an atmosphere having a temperature of 7 or higher.

Regarding the influence of water vapor on the firing of magnesia, P-J, Anderson et al., Trans., Far
adySoc, Vo160(5)930-937(64
): East-man et al. J, Am Soram, Vo.
Reports have been submitted by researchers such as 49(10') 526-530(66).

However, these studies mainly deal with the initial sintering process, and it cannot be said that sufficient consideration has been given to the gas composition including water vapor.

Conventional sintering of magnesia is actually carried out in an atmosphere having a significant partial pressure of water vapor.

That is, hydrocarbons such as heavy oil and natural gas are often used as fuels for firing magnesia, and as a matter of course, steam is produced when these hydrocarbons are combusted.

The present invention has discovered that when the composition ratio of water vapor and carbon dioxide gas exceeds 1.7, sintering of magnesia proceeds rapidly and the thickness of the crystal grains also increases. It is obtained by spraying steam at the highest possible temperature into an electric furnace or a furnace that burns hydrocarbons.

Although the influence of gases other than carbon dioxide is not as pronounced as carbon dioxide, some adverse effects are still observed, so it is desirable to minimize the influence.

Water vapor content is 25vol at total pressure 200mtHg or less
% or less, the effect of the present invention could not be obtained regardless of other gas content compositions.

It is particularly desirable that the water vapor content be 50vol% or more.

According to the method of the present invention, the firing temperature of magnesia can be significantly lowered.

That is, according to the method of the present invention, when firing is performed at a temperature of 1400°C or higher, it is possible to obtain a product comparable to the firing temperature of about 2000°C in the conventional method.

In magnesia firing, there is a composition suitable for the firing method.

In the case of the present invention, Ca00.03% or less5in20.0
Magnesia with a composition of 9 to 0.20%B2030.05% or less showed particularly good sintering properties.

The embodiments of the present invention are as follows.

Undecomposed raw materials such as magnesium carbonate and magnesium hydroxide can also be used as raw materials for magnesia, but M
If the gO density is 1.3 g/i or more, sintering tends to proceed rapidly, so it is particularly advantageous to use light sintered magnesia as a starting material.

When light burnt magnesia is used, it is desirable that the crystallite size of the light burnt magnesia is 0.1 μm or less because if it decomposes at too high a temperature, it will lose its activity.

The magnesia raw material is formed by a method such as double roll or tablet casting, and then fired in the above atmosphere.

More specifically, the molded body is fed into a firing furnace, and at the same time as heating, a gas containing water vapor is fed, and the temperature is raised to 1400° C. or higher while maintaining the partial pressure of the water vapor.

After reaching the specified temperature, it is desirable to hold the temperature under that water vapor partial pressure for at least 1 hour.
It is desirable to keep the temperature below 0°C/Hr and avoid rapid temperature rise.

The gas entrained with water vapor may normally be air, but preferably N2. An inert gas such as Ar is better.

This is because crystal growth is greater with inert gas even if the water vapor content is lower.

In any case, the water vapor content is particularly preferably 50vol% or more.

The temperature at which steam starts to be fed is preferably when the temperature of the fired product is low, but there is no big difference up to about 1000℃, and it is 1260℃.
Even if it is sent after reaching the above limit, it will have no effect.

As mentioned above, magnesia is often fired using heavy oil or natural gas as fuel, but due to the steam produced at this time, the steam content of the combustion gas is 10 to 20 when air is used.
If pure oxygen is used, the water vapor content can theoretically be 40 to 50 vol.

However, when hydrocarbon fuel is used, the crystal grain size is only a few microns when fired at about 1500℃! It is difficult for the crystal to grow even though the partial pressure of water vapor is high.

Regarding these, the present inventors investigated the relationship between the partial pressure of carbon dioxide gas in the firing gas and the partial pressure of water vapor, and found that the partial pressure of water vapor was at least 1.7 times the partial pressure of carbon dioxide gas, that is, N2CO2-
It was confirmed that the present invention is effective when the water vapor content is 25 vo 1% or more and is 1.7 times or more the partial pressure of carbon dioxide gas in an N20 system.

The present invention will be explained below with reference to Examples.

Example 1 Magnesium hydroxide produced by reacting seawater with slaked lime was purified, its composition was adjusted, and the composition was converted into calcined material.

A sample of raw material symbol A in Table-1 was obtained.

The above magnesium hydroxide was lightly calcined in a calcining furnace, and the crystallite size was adjusted to 350A by the X-ray half width method.

This lightly calcined magnesia was molded using a molding machine to form a molded body having a bulk specific gravity of 1.85.

The molded body was placed on a magnesia base plate and sent into an annular electric furnace. Initially, the temperature was raised to 800°C at a rate of 100°C/Hr, and when it reached 500°C, steam regulated by a steam generator was released. Sent.

The temperature was raised from 800°C at a rate of 200°C/Hr, and after reaching a predetermined temperature, it was maintained for 0.5 to 4Hr.

During this time, steam was released from the hearth and new steam was constantly supplied from the other hearth at a furnace passing rate of 2 CrIL/5 ec (converted to room temperature).

The bulk specific gravity of the fired body was measured by a conventional method, and the size of the periclase crystal grains was determined by the Fullman method from a photograph taken with a reflection microscope, and 200 or more grain boundaries were counted to obtain the average grain diameter.

Table 2 shows the results.

Comparative Example 1 The composition of magnesium hydroxide produced in the same manner as in Example 1 was adjusted to obtain samples shown in Tables B and C below in terms of calcined product.

The above magnesium hydroxide was lightly calcined in a calcining furnace, and the crystallite size was adjusted to 300 to 350 by the X-ray half width method.

This lightly calcined magnesia was calcined and measured in the same manner as in Example 1.

Table 4 shows the results.

Example 2 A molded body was prepared using a sample having the raw material code of Example 1 in the same manner as in Example 1, and fired in a propane furnace.

To adjust the gas composition, 1 mol of propane is combusted using a flowmeter at a ratio of 24 mol of air, while a steam generator is charged with 3 mol of air.
．． 180℃~ generated by supplying water at a ratio of 3 mol
Steam at 200°C was introduced into the furnace.

The temperature increase rate was adjusted by the amount of fuel gas, and at the same time, the amount of air and steam fed was varied at the above rate.

The temperature increase rate was approximately 300°C/Hr.

After reaching a predetermined temperature, the IHr was maintained at the same temperature.

Measurement of the sintered body was performed in the same manner as in Example 1.

Table 5 shows the results. Comparative Example 2 A molded body was prepared from a sample of raw material code A of Example 1 in the same manner as in Example 1, and fired in a propane furnace.

The same fuel feeding method as in Example 2 was used, but water vapor was not fed.

** The temperature increase rate was adjusted by the amount of fuel gas, and was approximately 300° C./Hr.

After reaching a predetermined temperature, the IHr was maintained at the same temperature.

Measurement of the sintered body was performed in the same manner as in Example 1.

Table 6 shows the results.

[Brief explanation of drawings]

FIG. 1 is a photograph of the Vericlese crystal grain size of Experiment I65 in Example 1 observed with a reflection microscope. FIG. 2 is a photograph of the periclase crystal grain size of the experimental layer 12 of Comparative Example 1 observed with a reflection microscope. Figure 3 is a photograph taken with a scanning electron microscope after intense etching with nitric acid to clarify the grain boundaries of the Vericlese crystal grains of I67 in the experiment of Example 1. 4 shows the experimental layer 15 of Comparative Example 1.
This photograph was taken with a scanning electron microscope after it was strongly etched with nitric acid to make the grain boundaries of the periclase crystal grains clearer, and shows a representative part of the inside of the fired product.

Claims (1)

[Claims] 1. A molded body of magnesium oxide is heated to a total pressure of 200 mmHg.
A method for firing magnesia, which comprises firing in an atmosphere containing 25 vol % or more of water vapor and having a ratio of water vapor partial pressure to carbon dioxide gas partial pressure (H20/C02) of 1.7 or more. 2. The fired magnesia old island 3 5102 t Cab and B2O3 content of 0.04 to 0.30% by weight and 0.2% by weight, respectively, as set forth in claim 1, characterized in that the firing temperature is 1400°C or higher. 35% by weight or less,
and 0.1% by weight or less, and a magnesia molded body having an MgO content of 99.3% by weight or more. . 4. The calcined old island of magnesia according to claims 1, 2, and 3, characterized in that the bulk specific gravity of the compact is 1.3 or more.