JP2001348631A - Method for reducing chromium-containing oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an efficient and low-cost method for reducing chromium- containing oxide with gas to be generated reduced. SOLUTION: In this method for reducing chromium-containing oxide, a compact composed of an oxide and C, in which chromium oxide is contained by 5 to 60 mass% is irradiated with microwaves to heat the compact, and a part or the whole of the chromium oxide is reduced by the same treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing chromium-containing oxides efficiently and at a low cost with a small amount of generated gas.

[0002]

2. Description of the Related Art As a technique for carbon reduction of chromium oxide, a method using a fossil fuel as a heat source is widely known. For example, Japanese Patent Application Laid-Open No. Hei 5-247555 discloses a technique for smelting and reducing chromium ore powder using a top-bottom blow converter, and Japanese Patent Application Laid-Open No. 58-197208 discloses a fluidized bed pre-reduction furnace and a shaft furnace type smelting reduction furnace. A combined method for reducing loam ore is disclosed in Japanese Patent Application Laid-Open No. 61-13 / 1986.
No. 0726 discloses a method for reducing chromium ore pellets using a rotary kiln. Since all of these methods use the combustion heat of fossil fuel (carbon material or hydrocarbon-based gas) as a heat source, they generate a large amount of exhaust gas, require large-capacity exhaust gas treatment equipment, and require large capital investment. There was a problem. In addition, a large amount of dust is generated along with generation of a large amount of exhaust gas.For example, when reducing dust containing chromium oxide generated in stainless steel refining, a large amount of dust is generated again from the reduction furnace. It was necessary to install a multistage reduction facility in order to reduce the waste. Furthermore, in the case of a rotary kiln, if the temperature in the furnace is raised to 1400 ° C. or higher in order to increase the reduction ratio, there is a problem that the pellets are softened and the pellets adhere to each other, making the operation difficult. A typical example of a method using electricity as a heating method is disclosed in
There is an electric furnace as disclosed in -80236. However, in the case of an electric furnace, although the thermal efficiency is high while the electrodes are covered with the insert, there is a problem that the thermal efficiency after melting is extremely low.

On the other hand, Japanese Patent Application Laid-Open No. 11-217615 discloses a method of reducing carbonaceous material-containing pellets using a rotary hearth furnace. However, the object product is iron and reduction of chromium oxide is described. It has not been. This is because, in this method, since the heating temperature is limited, it is generally 1400
This is because it is difficult to reduce chromium oxide which needs to be heated to a high temperature of not less than ℃.

One of the inventors of the present invention disclosed in Japanese Patent No. 2949221 a steel-based slag mixed with a carbon-based reducing agent, heated by microwave irradiation, and heated to an iron and phosphorus content in the steel-made slag. Disclosed is a method for treating steelmaking slag by microwave heating, which is characterized by reducing and removing components. However, the steelmaking slag in this case is in a molten state, and there is no knowledge about hardly reducible chromium oxide reduction in a solid oxide state.

[0005]

SUMMARY OF THE INVENTION As described above, the present invention relates to Japanese Patent Application Laid-Open Nos. 5-247555 and 58-19.
No. 7208 and Japanese Patent Application Laid-Open No. 61-130726 disclose the problem that a large amount of exhaust gas is generated and large-capacity exhaust gas treatment equipment is required because the combustion heat of fossil fuel is used. Also, there is a problem that a large amount of exhaust gas generates a large amount of dust.
In the technology disclosed in Japanese Patent Application Laid-Open No. 726-802, there is a problem that when the temperature in the furnace is increased, the pellets are softened and the pellets adhere to each other, making the operation difficult.
36, a problem that the thermal efficiency after melting is extremely low.
Japanese Patent No. 2994921 discloses that the reduction of irreducible chromium oxide is difficult due to the limitation of the heating temperature in the technology disclosed in Japanese Patent Publication No. 5 (1993) -175, and the carbon-based steel slag disclosed in Japanese Patent No. 2949221. It solves the problem that there is no knowledge about irreducible chromium oxide reduction in the solid oxide state in the technology of mixing a reducing agent and heating by microwave irradiation. An object of the present invention is to provide a method that enables low-cost reduction of a chromium-containing oxide.

[0006]

The gist of the present invention resides in the following methods. (1) An oxide containing 5 to 60% by mass of chromium oxide and C
A method for reducing a chromium-containing oxide, comprising: irradiating a microwave to a molded body comprising: heating the molded body; and reducing a part or all of chromium oxide by the treatment. Here, as the chromium-containing oxide,
Chrome ore, stainless steel smelting slag, dust generated in stainless steel smelting, and sludge generated during casting and rolling of stainless steel. Further, the molded body means pellets and briquettes, and the briquettes include compression molded ones. (2) In (1), the microwave frequency is set to 0.5 G
Hz or more and 30 GHz or less. (3) In (1) and (2), in the molded body composed of oxide and C, the mass ratio of oxide to C is 1 / C / oxide.
A method for reducing a chromium-containing oxide, which is 2/7/5. (4) The method for reducing a chromium-containing oxide according to any one of (1) to (3), wherein the particle diameters of the oxide powder and the C powder are 200 μm or less. (5) In (1) to (4), a compact formed of an oxide powder and a C powder is used for a compact made of oxide and C, and the pressure at the time of compression molding of the compact is 10 to 200 MPa.
a method for reducing a chromium-containing oxide. (6) In (1) to (5), the molded article is set to 1400 to 15
A method for reducing a chromium-containing oxide, comprising maintaining the temperature at 50 ° C. for 10 to 30 minutes. (7) The method for reducing a chromium-containing oxide according to any one of (1) to (6), wherein the compact is heated in an inert gas atmosphere. (8) The method for reducing a chromium-containing oxide according to any one of (1) to (7), wherein the molded body is placed on a rotating disk, and the molded body is reduced by passing through a microwave heating zone. (9) The method for reducing a chromium-containing oxide according to (1) to (7), wherein the molded body is charged into a rotating cylindrical furnace, and the molded body is reduced by passing through a microwave heating zone. .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Microwaves generate uniform heat by directly heating the inside of a substance. In addition, since the temperature of the substance to be heated itself is the highest, the heat load on the refractory container containing the substance is high. It has the characteristic of being small. There are two heating mechanisms when a material is irradiated with microwaves. The first is a mechanism in which, when a dielectric material is irradiated, a part of microwave electromagnetic wave energy is absorbed (so-called dielectric loss) and heat is generated by the energy. Second, when a conductor is irradiated with microwaves, surface current flows on the surface of the conductor, and heat is generated by losing the energy of the surface current as Joule heat in the conductor.

The present inventors have utilized the characteristics of the microwave to hybridize the properties of an oxide which is well-dielectrically heated at a high temperature of about 800 ° C. or higher and the Joule heating of carbon as a conductor. It has been found that microwave heating can be efficiently performed at a high temperature even in a solid state.

FIG. 1 shows the basic experimental results. A chromium ore (powder of 20 μm or less) and carbon (powder of 50 μm or less) were mixed at a mass ratio of 1 to 1 and molded at a compressive strength of 50 MPa ( Place about 24 g) in a heat-insulated furnace.
When irradiated with microwaves of 600 W and 2.45 GHz, the temperature could be raised to 1500 ° C. or more in only 30 seconds. As a result, Cr ₂ O ₃ in the sample was about 9 before and after the experiment.
5% was reduced to produce metallic Cr. The reduction ratio is a representation of the ratio of Cr ₂ O ₃ mass contained in the molded body before the experiment and Cr ₂ O ₃ by weight, which is reduced in weight percent, reduced Cr ₂ O ₃ mass after experiment Was calculated in consideration of the mass of C consumed by the reaction of the formula (1) from the mass of the molded product of the formula (1).

The reaction between Cr ₂ O ₃ and C is described by equation (1).

Cr ₂ O ₃ + 3C (S) = 2Cr (S) + 3CO (g) (1) The equilibrium constant is expressed by equation (2).

Log K = −38555 / T + 6.993 (2) Here, when the activity of each of C, Cr and Cr ₂ O ₃ is 1, the CO partial pressure necessary for the progress of reduction is 1500. 1 ° C
It is calculated as 0-5. Therefore, thermodynamically, Cr ₂ O
_The reduction hardly occurs unless the activity is lowered by melting ₃ or the generated Cr is dissolved in molten iron to lower the Cr activity. However, according to experiments, 95%
A degree of reduction had occurred. This is presumably because the atoms in the oxide were activated by the microwave to promote the reaction. The effective use ratio of microwave energy for heat reduction was as high as 90%.

Hereinafter, the present invention will be described in detail with reference to the claims.

[0014] Claim 1 is that the chromium oxide is 5 to 60% by mass.
A chromium-containing oxide characterized in that the molded body comprising the oxide and C is heated by irradiating the molded body with microwaves, and a part or all of chromium oxide is reduced by the treatment. It is a reduction method. The reason for containing 5 to 60% of chromium oxide is that if it is less than 5%, there is no economic value of consuming and reducing energy,
If it is more than 60%, the amount of heat absorbed by the reduction increases, so that the capacity of the microwave transmitter increases and the investment increases. The remainder other than chromium oxide may be iron oxide, MgO, Al ₂ O ₃ , CaO, SiO ₂ or the like. Specifically, it is desirable that the ratio of CaO / Al ₂ O ₃ be 1 or more in terms of mass percentage, and the ratio of CaO / SiO ₂ be 2 or more in terms of mass percentage. The reason for the molded body is that chromium oxide, which is dielectrically heated in order to efficiently promote the reduction reaction and temperature rise, and C, which is a reducing agent which is heated by joule heating,
This is for bringing into contact sufficiently with. It is better to reduce all the chromium oxide in the molded body, but the reduction time is long and the temperature is high, so even a partial reduction is economical in terms of productivity and thermal efficiency. In some cases. However, if the reduction rate is too low, the economic effect is small, so that the reduction rate is preferably 65% or more. The brand of C as the reducing agent is not limited as long as it is a carbon-containing substance typified by coke, anthracite, graphite and the like.

Claims 2 to 4 relate to a specific method for reducing a chromium-containing oxide using the technique of claim 1.

A second aspect of the present invention relates to a microwave, wherein the frequency of the microwave is 0.5 GHz or more and 30 G or more.
Hz or less. When the frequency is lower than 0.5 GHz, the heating efficiency is deteriorated because the basic characteristics as a microwave are reduced. When the frequency is higher than 30 GHz, only the surface portion of the sample is heated because the penetration depth is small.

A third aspect of the present invention is a molded article comprising an oxide and C, wherein the mass ratio of the oxide and C is 1/2 to 7 / C / oxide.
5 is assumed. In order to increase the heating rate in microwave heating, it is necessary to increase the mixing ratio of C to increase the heating rate in the low-temperature region. The amount of heat increases, the heating temperature does not increase, and the reduction rate decreases. Therefore, from the experimental results in FIG. 2, it is necessary to set the mass ratio of chromium oxide to C to 1/2 to 7/5 for C / oxide.

According to a fourth aspect, the particle diameter of the oxide powder and the C powder is set to 200 μm or less. In the compact, the particle size of the oxide powder has a large effect on the contact area, so the smaller the particle size, the higher the reduction rate. Further, since the surface area is increased by making the carbon powder finer, the Joule heating efficiency due to the surface current is increased. The lower limit of the powder particle size does not need to be particularly defined, but is determined only by the correlation between the increase in the raw material price accompanying the pulverization and the improvement in efficiency. When it is larger than 200 μm, the reduction ratio naturally decreases because the contact area becomes small.

According to a fifth aspect of the present invention, there is provided a molded article comprising an oxide and C, which is obtained by compression-molding an oxide powder and a C powder, and wherein the pressure during the compression molding of the molded article is set to 10 to 200 MPa. In order to efficiently reduce the chromium oxide with C, it is necessary to make the contact between the chromium oxide and C dense, so that a compact is required. Further, as shown in Table 1, in the compact, increasing the compression molding strength has a large effect on the contact area between chromium oxide and C, so that the greater the increase, the higher the reduction rate. In addition, carbon powder is liable to crack at the time of compression. Therefore, if the compressive strength is increased, the surface area increases, so that the Joule heating efficiency increases. Pressure during compression molding is 1
If it is less than 0 MPa, the above effect cannot be obtained, and the reduction rate is low. If it is more than 200 MPa, the reaction efficiency decreases because the number of pores for the reaction product CO gas to escape from the molded body decreases. I do.

[0020]

[Table 1]

According to a sixth aspect of the present invention, the molded body is formed at 1400 to 1550 ° C.
At a temperature of 10 to 30 minutes. Chromium oxide requires high temperature due to its hardly reducing property. As shown in FIG. 3, the temperature is 1400 ° C. or more and the reduction time is 1 hour.
0 minutes or more are required. When the temperature rises to 1400 ° C. or higher, the surrounding refractory becomes hotter than the molded body in the case of combustion heating with normal fossil fuel, but in the case of microwave, the temperature of the molded body itself is the highest. Due to high temperatures, refractory erosion is extremely small. However, if the temperature is higher than 1550 ° C., the refractory will be damaged. Even if the holding time is longer than 30 minutes, the reduction rate hardly increases, so that it is 30 minutes or less in consideration of energy efficiency.

According to a seventh aspect, the compact is heated in an inert gas atmosphere. Ar as an inert gas,
Nitrogen, CO, H ₂ is used. By using an inert gas atmosphere, the partial pressure of the CO gas is reduced to increase the reduction rate, and the reduction of the reduction rate due to the re-oxidation of the generated Cr can be prevented. Here, the inert gas atmosphere does not require that the oxygen concentration in the gas phase be completely zero, and it is sufficient if the concentration is such that the reoxidation of Cr can be suppressed.

An eighth aspect of the present invention specifically defines a reduction furnace, in which a molded body is arranged on a rotating disk, and the molded body is heated and reduced by passing through a microwave heating zone. An example of a specific embodiment is shown in FIG.
A pellet or pressure-molded briquette (b) is arranged on the rotary hearth (b), and a heating zone (c) whose periphery is covered with a refractory is installed on a part of the rotary hearth, and the heating zone is The microwave generated by the microwave oscillator (d) is transmitted to the waveguide (e)
And irradiate the molded body in the heating zone.

According to this method, since the generated gas is only the CO gas generated by the reduction reaction, a small exhaust gas treatment facility is sufficient.

A ninth aspect of the present invention specifically defines a reduction furnace. The reduction furnace is charged into a rotating cylindrical kiln (rotary kiln), and the molded body is heated by passing through a microwave heating zone. It is a method of reducing. An example of a specific embodiment is shown in FIG. 5, which is a rotating cylindrical furnace (a).
And pressurized briquettes (b) are charged into the furnace, and the microwave generated by the microwave oscillator (c) is guided through the waveguide (d) into the cylindrical furnace, and the molded body in the heating zone is introduced. Irradiation. According to this method, since the generated gas is only the CO gas generated by the reduction reaction, a small exhaust gas treatment facility is sufficient. Furthermore, during combustion heating using fossil fuel, if the heating temperature is increased to increase the reduction rate, the surface temperature becomes higher than that inside the molded body, so that when the inside is heated, the surface is always softened. On the other hand, since the microwave can uniformly heat the inside and the surface of the molded body, the surface temperature is significantly lower than that during the combustion heating, and heating to a high temperature is possible without softening.

[0026]

EXAMPLE 1 In Example 1, a test apparatus shown in FIG. 6 was used. That is, chromium oxide-containing steelmaking slag (powder of 100 μm or less) and carbon (powder of 50 μm or less) are mixed at a mass ratio of 1: 1 in a box furnace (a) insulated with a refractory, and compressed with a compressive strength of 50 MPa. After molding, briquettes (b) crushed to about 10 mm are placed in about 2 kg, and microwaves of 1.5 kW and 2.45 GHz are irradiated into the furnace from a microwave oscillator (c) via a waveguide (d). did. The atmosphere in the furnace was an Ar atmosphere, and the oxygen concentration was 0.1% or less. Table 2 shows slag composition
Shown in

[0027]

[Table 2]

Within about 1 minute after the start of the microwave irradiation, the temperature of the briquette reached 1500 ° C., and the reduction rate of Cr was 91% by the heat treatment for 20 minutes. The input power is 9
It was 8 kW and the heating efficiency was as high as 91%. The exhaust gas was only about 350 Nl mainly composed of CO gas produced by reduction.

Example 2 In Example 2, a cylindrical furnace test apparatus shown in FIG. 5 was used.
That is, a rotating cylindrical furnace (a) insulated with a refractory was charged with briquettes (b) having the same composition and shape as those of Example-1 for about 2 k.
g, 1.5 kW, 2.45 GHz microwave,
Irradiation was performed from a microwave oscillator (c) into the furnace via a waveguide (d). The atmosphere in the furnace is Ar atmosphere and the oxygen concentration is 0.1
% Or less. By a heat treatment at 1400 to 1550 ° C. for 20 minutes in a temperature range, the reduction ratio of Cr was 88%. The input power is 103 kW and the heating efficiency is 87
% Was high. The exhaust gas was only about 350 Nl mainly composed of CO gas produced by reduction.

Example 3 In Example 3, a rotary hearth test apparatus shown in FIG. 4 was used. That is, a briquette (b) having the same composition and shape as in Example-1
Approximately 2 kg is placed on a rotary hearth (a), and in a heating zone (c) insulated with a refractory, a 1.5 kW, 2.45 GHz microwave is transmitted from a microwave oscillator (d) to a waveguide (e). Irradiation via The heating zone was an Ar atmosphere and the oxygen concentration was 1% or less. The time required to pass through the heating zone in the temperature range at 1400 to 1550 ° C. was about 15 minutes, but the reduction ratio of Cr was 87%. The input power was 101 kW, and the heating efficiency was as high as 88%. The exhaust gas was only about 350 Nl mainly composed of CO gas produced by reduction.

Comparative Example The same test apparatus as in Example 1 was used as a comparative example, except that an LPG / oxygen burner was installed at the waveguide installation position,
Was heated at a flow rate of 1.25 Nl / min. It took about 5 minutes to reach 1500 ° C. after the start of heating, and the heat treatment for 20 minutes tended to soften the briquette surface. Although the reduction ratio of Cr was 88%, the heating efficiency was 66%.
% Was low. Exhaust gas was as large as about 750 Nl including CO gas from burner combustion.

[0032]

According to the present invention, an efficient and low-cost reduction of chromium-containing oxides can be carried out with little generated gas.

[Brief description of the drawings]

FIG. 1 shows an example of a temperature rise behavior of a chromium oxide-containing oxide and a C molded product by microwave heating.

FIG. 2 is an experimental result showing a relationship between a mass ratio of oxide and C (C / oxide) of a molded body and a reduction ratio.

FIG. 3 is an experimental result showing the effect of temperature on the reduction rate of a compact.

FIG. 4 is an example showing an embodiment of the present invention, wherein A is a rotary hearth, B is a pellet or pressure-formed briquette, C is a heating zone whose periphery is covered with a refractory material, and D is a microwave. Oscillator and E show a waveguide.

FIG. 5 is another example showing the embodiment of the present invention, wherein A is a cylindrical furnace, B is a pellet or pressure-formed briquette, C is a microwave oscillator, and D is a waveguide.

FIG. 6 is another example showing the embodiment of the present invention, wherein a is a box furnace insulated with a refractory, b is a compression-molded briquette of chromium oxide-containing steelmaking slag and carbon, and c is a microwave oscillator. , D indicate a waveguide.

Claims (9)

[Claims] 1. A molded body comprising an oxide containing 5 to 60% by mass of chromium oxide and C is irradiated with microwaves to heat the molded body, and a part or all of the chromium oxide is subjected to the treatment. A method for reducing a chromium-containing oxide, comprising reducing chromium. 2. The method for reducing a chromium-containing oxide according to claim 1, wherein the frequency of the microwave is 0.5 GHz or more and 30 GHz or less. 3. A chromium according to claim 1, wherein the mass ratio of oxide and C is 1/2 to 7/5 in terms of C / oxide in the molded body comprising oxide and C. Method for reducing contained oxide. 4. The method according to claim 1, wherein the oxide powder and C
A method for reducing a chromium-containing oxide, wherein the particle size of the powder is 200 μm or less. 5. A compact according to claim 1, wherein said compact comprises an oxide powder and C powder and is obtained by compression-molding an oxide powder and a C powder.
A method for reducing a chromium-containing oxide, which is performed at 0 MPa. 6. The molded article according to claim 1, wherein
A method for reducing a chromium-containing oxide, wherein the method is maintained at a temperature of 1550C for 10-30 minutes. 7. The method for reducing a chromium-containing oxide according to claim 1, wherein the compact is heated in an inert gas atmosphere. 8. The method for reducing a chromium-containing oxide according to claim 1, wherein the molded body is placed on a rotating disk, and the molded body is reduced by passing through a microwave heating zone. 9. The method for reducing a chromium-containing oxide according to claim 1, wherein the molded body is charged into a rotating cylindrical furnace, and the molded body is reduced by passing through a microwave heating zone. .