JPS5935965B2 - Calcium carbide-alkaline earth metal halide dephosphorization flux stabilization method - Google Patents

Description

Detailed Description of the Invention The present invention relates to a calcium carbide-alkaline earth metal halide flux, such as CaC2-CaF2, which is used to dephosphorize molten metal that is difficult to dephosphorize (for example, high-chromium molten metal) by a normal oxidation refining method. This invention relates to a method for stabilizing system flux.

Hot metal and molten steel of normal composition can be easily dephosphorized by oxidative refining in the presence of basic slag.

On the other hand, when oxidation refining is performed on molten metal containing large amounts of easily oxidized elements such as chromium and silicon, a large amount of these valuable elements will be oxidized, and they will enter the slag and physically and chemically impair its dephosphorizing ability. Conventional dephosphorization methods were considered impossible because of the weakening of

However, in recent years, the quality of the raw materials for ferrochrome production, such as coke and chromium ore, has deteriorated, and the amount of phosphorus in particular has been on the rise. is on the rise.

On the other hand, the upper limit of the phosphorus content of stainless steel materials, which are final products, is increasingly being strictly regulated in terms of material quality.

Therefore, there is a strong desire to be able to dephosphorize high-chromium molten metal.

As a method to enable dephosphorization of high-chromium molten metal, the present inventors first developed a carbon-unsaturated molten metal and CaC2゜CaF2
He invented a method for reacting a flux containing as a main component under conditions where the atmosphere is sealed from the atmosphere and the supply of oxygen from the slag is suppressed (Japanese Patent Application No. 50-2943, Patent Application No. 1983- 99143).

In this method, by selecting various conditions such as the carbon content of the molten metal, the flux component and the amount used, it is possible to perform arbitrary dephosphorization even on high-chromium molten metal.

This method is completely different in principle from conventional dephosphorization by oxidative refining, and should be called ``reductive dephosphorization.''

That is, when carbon unsaturation, molten metal, and flux mainly composed of calcium carbide and calcium fluoride are reacted in a non-oxidizing atmosphere, -(Ca C2) → (Ca) +2 [C]・”=
(1) yields free metallic calcium, then 3 (
Ca) + 2 CP]→(Ca3P2)---
-・-(2) reaction causes dephosphorization of the molten metal.

In implementing this process, it is important to prevent phosphorus from returning to the molten metal after dephosphorization and to stabilize the separated flux.

In other words, if the atmosphere becomes oxidizing after dephosphorization treatment, the metallic calcium in the flux will be oxidized and consumed (2)
The reverse reaction of the formula proceeds and rephosphorus occurs.

In order to avoid this, it is necessary to separate the molten metal and flux as completely as possible after the dephosphorization reaction is completed.

Furthermore, if this separated flux is cooled in the atmosphere, there is a risk that phosphorus compounds in the flux will react with water or water vapor to generate phosphine (PH3).

Ca 3P2) + 3H2G → 3(CaO) + 2PHs↑
-(3) Since phosphine is a poisonous and odorous gas, it is essential to stabilize the flux so that the reaction in equation (3) does not proceed to the right from the environmental point of view.

For this purpose, the flux can be oxidized.

That is, (Ca3F2)+40□-+3 (CaO)+P
If the phosphorus in the flux is converted into an oxide form through the reaction of 2O5''.(4), the generation of phosphine due to the reaction with moisture can be prevented.

However, in actual processes, it is not easy to completely stabilize the flux and prevent it from generating phosphine.

This is because if the flux is kept in a molten state, it is possible to oxidize the phosphorus almost completely in a relatively short period of time and stabilize the flux by spraying oxidizing gas. When separated from the liquid, it is inevitable that a portion of it will solidify due to heat dissipation.

When partially solidified flux is exposed to an oxidizing atmosphere, the oxidation reaction proceeds from the surface and the oxidized layer forms a shell, which acts as resistance to oxidation and causes a phenomenon in which the oxidation reaction rate decreases. .

Therefore, when heat is dissipated in the air or an oxygen atmosphere, it is impossible to completely reoxidize the bulk flux due to the temperature pattern.

If only the surface layer of the flux is oxidized, it can be temporarily stabilized against water vapor in the atmosphere, but when it gets wet with water, it is unavoidable that phosphine will be generated, and it will not be completely stable. I cannot say that it has been stabilized.

Additionally, during the dephosphorization process, some of the flux seeps into the refractory lining of the reaction vessel, and if this is not stabilized through oxidation, it may react with atmospheric water vapor and generate phosphine. It is an environmental problem.

As with the solidified flux described above, it is impossible to stabilize the flux that has permeated into the refractory by oxidation simply by normal cooling.

In view of the above circumstances, the present invention utilizes a calcium carbide-alkaline earth metal halide dephosphorization flux, for example, C.
aC2-CaF2-based dephosphorization flux (hereinafter referred to as CaC2-
This was obtained as a result of various studies aimed at stabilizing CaF2-based materials (including those that have permeated into the refractories of the reaction vessel) by oxidation, and the dephosphorization reaction is not performed. This method is characterized by allowing the flux to remain in the reaction vessel and heating and maintaining the interior of the reaction vessel with an oxidizing flame from a burner.

Hereinafter, a detailed description will be given based on specific examples.

The molten metal to be dephosphorized is a molten metal that is difficult to dephosphorize using normal oxidation refining methods, such as ferrochrome or stainless steel with a high chromium content.

Note that carbon unsaturation is required in order to produce free metallic calcium necessary for refining by the reaction of formula (1).

The flux used for dephosphorization contains two main components, CaO2 and CaF2, and is a mixture of calcium carbide and fluorite, for example.

According to equation (1), CaC2 in the flux is essential for reacting with carbon-unsaturated molten metal to produce metallic calcium (if the molten metal has a low degree of carbon unsaturation, CaC2
When a part of 2 is replaced with calcium or its alloy, the concentration of metallic calcium in the flux can be increased to make dephosphorization advantageous.

), CaF2 is necessary to adjust the melting point of the flux and to allow metallic calcium to stably exist in the slag.

The mixing ratio of CaC2 and CaF2 is CaC2/CaF2=
A range of 0.5 to 5 is optimal.

The remaining components are CaO1SiO2 and the like mixed in as impurities, but it is desirable that the amount of oxides that are easily reduced is as small as possible.

Such a flux and the above-mentioned molten metal are mixed under conditions where free metallic calcium in the flux is oxidized and consumed (
Specifically, the catalytic reaction is carried out in a situation where the atmosphere is sealed from the atmosphere and the supply of oxygen from mixed slag and molten metal is suppressed.

Various forms of contact between the molten metal and the flux are possible, as in the case of normal slag refining, but since the flux of the present invention has a high CaF2 content and easily interacts with refractories, the time required for the reaction can be minimized. It is necessary to devise ways to shorten the time.

For this purpose, it is effective to use, for example, the stirring action when transferring the molten metal, or to perform stirring by blowing gas into the molten metal.

After dephosphorization is performed by promoting contact between the molten metal and flux under controlled atmosphere, the flux and molten metal are separated under conditions that prevent phosphorus from occurring as much as possible.

At the end of the dephosphorization reaction, phosphorus in the flux mainly exists as a compound with Ca (Ca3P2).

The total phosphorus content in the flux varies depending on the conditions of the molten metal, flux, etc., but is approximately 0.2 to 3':l).

If the flux after dephosphorization and the molten metal are mixed in an oxidizing atmosphere such as the air, rephosphorization tends to occur (also, if the molten metal is decarburized by oxidation refining in the next process without sufficient separation of the flux) Since rephosphorus occurs in
The method of separating flux and molten metal requires some ingenuity.

In addition, in the method of removing slag by tilting a container as in the case of ordinary slag, there is a risk that the discharged slag will be scattered, and this method is not suitable for a flux that requires post-treatment, such as the flux of the present invention.

Therefore, for example, a suitable method is to attach a nozzle to the bottom of the reaction vessel used for dephosphorization, and after the dephosphorization reaction is complete, open the nozzle and let the molten metal flow out into another vessel, leaving the flux to remain in the reaction vessel. There is.

In this method, by closing the nozzle at an appropriate point,
All the flux used for dephosphorization (including the one that adhered to the refractory lining of the reaction vessel and the one that permeated) -
Since the flux can be collected in one place, it is easy to stabilize the flux in the next process.

Although a small amount of the flux separated from the molten metal as described above (some of it has solidified), it is then heated with an oxidizing flame from a burner in order to oxidize and stabilize it.

Note that it is desirable that the flux temperature does not fall below 200° C. after the completion of the dephosphorization reaction and before heating with a burner.

At temperatures below 200℃, it reacts with the moisture in the atmosphere (3)
This is because there is a risk of generating phosphine due to the formula.

FIG. 1 shows the situation when the interior of the reaction vessel is heated with a burner 3 as a post-treatment when the flux 2 is left in the reaction vessel 1 as described above.

As equipment, for example, equipment normally used for preheating a ladle or the like can be used as is.

Cover the reaction vessel with cover 4, and apply flame 5 from burner 3.
to occur.

The fuel may be heavy oil, hydrocarbon-based gas, or CO-based gas A, but as mentioned above, the time from the completion of the dephosphorization reaction to the start of burner heating is long and the temperature of the flux is below 200°C. If there is a risk of
In order to stop the generation of phosphine, it is desirable that the water vapor partial pressure in the combustion atmosphere be as low as possible, that is, use a CO-based gas.

Combustion air can be atmospheric air, oxygen-enriched air, pure oxygen, etc.
It may be either.

In the figure, 6 indicates a hopper for supplying iron powder, oxygen iron powder, silica, etc., and 7 indicates a sliding nostle.

It is necessary that the flux can be heated and maintained at a temperature of 600° C. or higher by heating with a burner.

The reason for this is that, as shown in Figure 3, the thickness of the layer stabilized by oxidation on the surface of solid flux is a function of temperature, but when the temperature is below 600°C, the rate of stabilization is small. This is because holding for several tens of hours or more is required to stabilize the center, which is not practical.

Also, (Ca3P2)+02→(CaO)+(P2O,)”=
As the oxygen source for oxidizing according to (5), it is suitable to use excess oxygen for burner combustion, that is, to use an oxidizing flame.

Although heat and oxygen may be supplied separately, it is more advantageous to use an oxidizing flame both thermally and reactively.

Figure 2 shows the excess oxygen ratio of the burner (the theoretical amount of oxygen required for complete combustion is assumed to be 1.0).

) and the ratio of remaining Ca3P2 out of the total 97 minutes in the flux. This figure shows that in order to sufficiently stabilize the flux, it is necessary to increase the oxygen excess ratio to 1.1 or more. It can be seen that it is desirable to set the value to 1.2 or more.

Furthermore, according to equations (5) and (6), if the flux is surrounded by a dense shell, the oxidation reaction rate of the flux decreases.

In addition, since the CaO content is high, dandruff tends to occur after treatment.

As a countermeasure against this, it is desirable to perform burner heating while adding silica or iron oxide.

As an efficient embodiment, there is a method in which a small amount of iron oxide powder, iron powder, silica powder, etc. is mixed with gas in a part of the burner and the mixture is sprayed onto the flux together with the flame.

In that case, it is desirable to rotate the burner and spread the above powder on each part of the flux surface. This has the advantage of simultaneously stabilizing the flux that has soaked into the refractory.

After performing the above treatment to oxidize the phosphorus content in the flux to an acceptable level (the concentration of phosphoric acid present as phosphoric oxides is reduced to 0.05% or less).

), reactor vessel (reaction vessel is blocked and slag is discharged).

The slag thus obtained is stable against atmospheric water vapor and can be disposed of or added to a steelmaking furnace as an auxiliary raw material.

By carrying out the present invention as described above, the flux after dephosphorization of high chromium molten metal can be rationally and reliably stabilized.

In particular, this stabilization treatment can be carried out using existing equipment, that is, equipment for preheating the reaction vessel, which is of great practical value.

Example (1) Stainless steel crudely molten steel (C: 0.61%) in an electric furnace.

Cr: 18.0%, Ni 2 7.9%,
P: 0.028o, S: o, 019%)
The hot water is poured into a reaction vessel (ladle with a lid) in which the atmosphere is kept non-oxidizing through a molten ILL and a honey rattle, and at that time, powdered flux (CaC2: 45%-CaF2
"40'fbpCaO: 12L:ib) to 5A
-9/ is added to perform dephosphorization.

The molten metal components after dephosphorization are C: o, 69% pCr: 1
8. O%, Ni: 8.0%, P: 0.013
%, S: 0.007%.

After tapping, the sliding nozzle provided at the bottom of the reaction vessel is opened to allow the molten metal to flow out into another ladle, and before the flux begins to flow out, the sliding nozzle is closed to allow the flux to remain in the reaction vessel.

Next, using the ladle preheating device in the reaction vessel, the cover is covered and the flux in the reaction vessel is heated by the oxidizing flame of the burner.

The heating conditions of the burner are as follows.

The average composition of the flux before and after this treatment is as follows.

By performing this treatment, no phosphine was detected in either the flux or the reaction vessel even when the reactor was cooled in the atmosphere.

[Brief explanation of the drawing]

Figure 1 shows an example of the situation when the flux remaining in the reaction vessel (ladle) is heated by a burner, and Figure 2 shows the excess oxygen rate of the burner (the amount required for complete combustion). Figure 3 shows the relationship between the theoretical oxygen content (assuming the amount of oxygen is 160) and the proportion of phosphorus in the flux that remains as Ca3P2. FIG. 3 is a diagram showing the influence of holding temperature. 1: reaction vessel, 2 varnish slag, 3: burner, 4: M,
5: Two flames, 6: Hopper, 7: Sliding nozzle.

Claims (1)

[Claims] 1 After carrying out a dephosphorization reaction using a flux whose main components are molten calcium carbide and alkaline earth metal halides, the flux is left in the reaction vessel used for dephosphorization, and the inside of the reaction vessel is exposed to the oxidizing flame of a burner. A method for stabilizing a calcium carbide-alkaline earth metal halide dephosphorization flux, the method comprising heating and holding the flux at