JPS6249346B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high chromium alloys from chromium ore at low cost and with a high chromium yield, without relying on electrical power.

Conventionally, high chromium alloys, such as ferrochrome containing 50% or more of Cr, have been produced by heating, melting, and reducing chromium ore or its semi-reduced product in an electric furnace. If this energy source could be replaced with primary energy that is cheaper than electricity (especially the combustion energy of coal-based solid carbonaceous substances such as coal and coke), it would have a huge economic advantage.

When attempting to manufacture a high chromium alloy such as ferrochrome by the smelting reduction method, there are two reasons why it is more difficult than smelting reduction of iron.

(i) Gangue content (MgO,
(Al ₂ O ₃ , SiO _{2 ,} etc.) is significantly higher than that of iron ore. The amount of slag produced when this gangue is turned into slag is 800 to 1000 kg/t.

(ii) Chromium oxide in chromium ore is MgO,
It combines with Al ₂ O ₃ etc. to form spinel and is difficult to melt. Therefore, the rate at which the Cr content (present as oxide or metal grains) in the slag usually decreases is slow. To melt spinel early, (a) increase the slag temperature, (b) lower the melting point temperature of the slag by adding flux, and reduce carbon content in the carbon material or metal bath of the molten slag. In order to increase the rate of reduction reaction by It is actually easy to obtain a high chromium alloy such as ferrochrome as a raw material at a relatively high rate without having a significant negative effect on the refractory unit consumption, because the appropriate conditions are generally contradictory. isn't it.

The present invention solves the above problems and reduces the Cr% in the discharged slag without relying on electricity.
To develop a process for manufacturing high chromium alloys that reduces the amount of refractory to the same level as the current electric furnace method (approximately 3%) or lower, while increasing productivity and keeping the unit consumption of refractories within an acceptable range. This was obtained as a result of various studies with the aim of In the smelting reduction smelting method for chromium alloys, after reducing more than 90% of the iron oxide in chromium ore using external coal in a preliminary reduction furnace, this pre-reduced chromium ore (chromium raw material) is transferred to the bottom (non-straight). A smelting stage in which bottom-blown acid is carried out while being charged into a top-bottom blown converter type melting reduction furnace in which the cross-sectional area of the body (straight part) is 2 to 5 times the average cross-sectional area of the part); Next, the method comprises a smelting stage in which the supply of the pre-reduced chromium ore (chromium raw material) is stopped, and blowing acid and stirring of the bath are carried out.

Hereinafter, a detailed explanation will be given using specific examples.

FIG. 1 shows an example of equipment used for smelting reduction smelting. 1 is a rotary kiln used for heating and solid phase reduction of a mixture (pellets, etc.) of chromium ore powder and carbonaceous powder such as coke. Reference numeral 2 denotes a converter-like melting and reduction furnace for receiving the pre-reduced pellets, melting the pellets, and proceeding with the reduction of the remaining chromium and iron oxides. From an operational standpoint, a special furnace interior shape as shown in the figure is required, which will be discussed later. The melting reduction furnace is equipped with a tuyere 3 (there may be a plurality of tuyeres) for blowing oxygen-containing gas from the bottom, and a lance 4 for blowing oxygen into the furnace from above. Note that 5 is a hood, and 6 is a hopper for carbon and lime.

The operating method is as follows. When starting operation, less than 50% of the rated amount of molten metal containing carbon obtained in a separate melting furnace is charged to the smelting reduction furnace.
As such a molten metal, hot metal or ferrochrome molten metal obtained from a smelting reduction furnace and kept warm in an induction melting furnace or remelted is suitable. Note that once the operation is started, the operation is repeated by leaving a part of the generated molten metal remaining when it is discharged from the furnace. Reducing the amount of molten metal as much as possible, or at least keeping the ratio of the remaining molten metal to 50% or less, is one of the factors that determines the internal shape of the smelting reduction furnace as described below.

A gas containing oxygen is supplied into the molten metal 7 from the bottom blowing tuyere 3 (for example, the tuyere is made of a double pipe, and the tuyere protective gas such as propane gas or Ar is supplied from the outer pipe, and oxygen gas is supplied from the inner pipe). Infuse. 11 is a bubble.
The effects of this bottom blowing gas are (i) strongly stirring the metal and the generated slag layer to increase the reduction reaction rate of chromium oxide, (ii) burning the carbon in the metal and heating the metal bath, 2. Maintaining the metal at an appropriate temperature (20℃ or more, 100℃ or less higher than the freezing point)
It is one. Regarding the latter, if there is no injection of oxygen-containing gas and there is a lot of slag, such as in ferrochrome smelting, the heat generated in the smelting reduction furnace 2 will be due to the slag 8 caused by the oxygen supplied from the top blowing lance 4. This will depend on the combustion of the carbonaceous materials inside to produce CO, or the secondary combustion reaction of the generated CO gas to _CO2 , and heat will be supplied to the metal through the slag, causing the slag temperature to change to the metal temperature. It will be necessary to make it higher. This is undesirable because the high slag temperature increases the load on the refractories.

The oxygen supply from the top blowing lance 4 is for burning carbonaceous materials such as coke to generate heat, and is the main source of heat generation in this smelting method. In order to increase the smelting reaction rate, it is particularly important to increase the combustion rate of the carbonaceous material by top-blown oxygen to increase the heat generation rate. For this purpose, it is desirable to make it easier for the oxygen jet blown into the carbonaceous material such as coke present in the furnace. For this purpose, it is necessary to make the lance nozzle 4 wide-angle and have multiple holes (for example, 7 holes) to widen the oxygen jet and to widen the area where the coke exists. The latter viewpoint is one factor that determines the internal shape of the melting reduction furnace 2, as will be described later.

In the stage of melt reduction, preheated and pre-reduced chromium raw material 9 (semi-reduced chromium pellets) is poured from rotary kiln 1 onto the remaining metal.
Perform top and bottom blowing acid while supplying melting and chromium,
Part of the reduction of iron oxides proceeds. Carbon materials such as coal and coke10 as reducing agents and exothermic agents
is added. Further, a flux mainly consisting of lime is added from the hopper 6 in order to slag the gangue content in the chrome pellets and the gangue content in the carbonaceous material.

Note that as a heat source for the rotary kiln 1, high-temperature gas containing CO and CO ₂ generated from the melting reduction furnace 2 is mainly used. However, during unsteady operation, the gas generated from the smelting reduction furnace may be temporarily stored in a tank, or an external heat source such as natural gas or heavy oil may be used. The reasons why it is necessary to pre-reduce chromium ore using a rotary kiln etc. are as follows: (i) By utilizing the latent heat and sensible heat of the gas generated from the smelting reduction furnace, the amount of heat required in the smelting reduction furnace can be reduced. to decline.

(ii) The poorly soluble chromium spinel, which is generally the cause of the slow reduction rate of chromium ore, is destroyed by preliminary reduction, increasing the reduction reaction rate and shortening the time required.

(iii) The reaction in the smelting reduction furnace is made more efficient by reducing the iron oxide concentration in the chromium ore through preliminary reduction, lowering the iron oxide concentration in the slag in the smelting reduction furnace, and reducing the corrosion resistance of refractories. This is extremely important in order to achieve this goal. Regarding the degree of preliminary reduction, regarding (i) above, the higher the preliminary reduction rate is, the more advantageous there is, and there is no clear limit condition. On the other hand, the preliminary return rates for (ii) and (iii) are shown in Figures 2 and 3.
It was found that the effect was as shown in the figure. In other words, in order to reduce the refractory consumption rate in the smelting reduction furnace by reducing the refractory erosion index and increasing the reduction reaction rate to shorten the total processing time, the preliminary reduction rate of chromium pellets should be , chromium content reduction rate of 60% or more, iron content reduction rate
It can be seen that it is necessary to increase the ratio to 90% or higher. Such a preliminary reduction rate is at a high level that is difficult to stably achieve with a normal rotary kiln. In the present invention, by taking advantage of the characteristics of the post-process (that is, since the reduction furnace is a smelting reduction furnace rather than an electric furnace, the amount of carbon material used is large), the amount of external coal charged in the rotary kiln is increased, and the rotary kiln is capable of unsteady operation. The above preliminary reduction rate conditions can be achieved by performing the following steps.

After charging a predetermined amount of semi-reduced chromium pellets, the smelting reduction moves to the third stage. In the third stage, semi-reduced chromium pellets are not supplied, and blowing acid and stirring are carried out (charcoal material is replenished as necessary).
Continue. In the second stage, unlike the first stage, there is no supply of chromium oxide into the smelting reduction furnace.
The amount of chromium present as an oxide in the slag decreases with processing time. The chromium percentage reached depends on the balance between the slag reduction rate and the reoxidation rate by blowing acid. Therefore, the lower the acid blowing rate, the lower the % chromium (present as oxide) in the slag. This value is 0.7-0.05%
However, in practice, a target chromium percentage at the end of the period is set from various points such as chromium yield, economic efficiency, and how to use the generated slag, and blowing is carried out to achieve it. Operating conditions such as acid pattern (in principle, the amount of blown acid is reduced continuously or stepwise) and treatment time are determined. Although it is not a necessary condition, it is also possible to combine a treatment for reducing the slag chromium percentage by electric heating or addition of a silicon-based alloy at the end of the smelting reduction stage.

The biggest technical challenge during the smelting and reduction stages is to find a way to increase the production rate of the smelting reduction furnace as much as possible without placing an excessive load on the refractories. Regarding the smelting reduction stage, the production rate is related to the reduction rate and preheating temperature of the pre-reduced pellets, the combustion rate of carbonaceous materials such as coke (blowing acid rate, furnace cross-sectional area, slag stirring speed), and the stage of smelting reduction. is the reduction rate of chromium oxide (amount of slag produced, excess amount of carbon material, slag temperature, slag stirring speed, etc.). On the other hand, factors related to the basic unit of refractories include the slag composition, the flow rate of slag near the refractories, temperature, and time. Some of the operational factors that increase production rates (e.g., increased slag temperature, increased slag agitation, etc.) have significant negative effects on refractories. Therefore, the problem cannot be solved simply by selecting operating conditions.
In contrast, the present invention takes the following method.

Regarding the internal shape of the smelting reduction furnace, the average cross-sectional area ( _S
is 1/2 to 1/5 of the cross-sectional area (S _S ) of the smelting reduction furnace body.
The depth of the molten metal should be at least 50 cm when 30% of the molten metal has entered.

The reason is as follows.

(i) Even if the amount of remaining hot water is as small as possible, the bottom blowing tuyeres can be protected and normal bottom blowing can be performed without blow-through (the conditions for this are that even with 30% remaining hot water, the depth is 50
cm or more).

(ii) Carbon materials such as coke react with oxygen mainly on the slag surface and generate heat through combustion. Therefore, the larger the surface area of the slag (that is, the area of the smelting reduction furnace barrel), the more the amount of blown acid can be increased and the rate of heat generation can be increased.

(iii) Stirring of the slag is mainly carried out by bottom blowing gas into the metal bath. The degree of slag agitation depends on the furnace geometry. Slag agitation affects both the rate of chromium reduction in the slag and the refractory consumption index. FIG. 4 shows the ratio of S _L and S _S described above: S _S /S _L as the reciprocal of the chromium reduction rate index in the slag (the time required for the Cr% in the slag to decrease from 10% to 1%). ) and its effect on the refractory consumption index. As the ratio of S _S /S _L increases, the effect of bottom-blown gas agitation on the flow of slag near the refractory becomes smaller, and the refractory consumption index becomes smaller. The strength becomes smaller and the reduction rate index also becomes smaller. Here, S _S means the cross-sectional area of the body of the smelting reduction furnace, and S _L means the cross-sectional area of the portion corresponding to the remaining hot water at the bottom of the smelting reduction furnace.

In order to reduce the consumption of refractories and at the same time keep the reduction reaction rate at a high level, S _S /S _L
It can be seen that the ratio of is required to be between 2 and 5.

There are two ways to change the cross-sectional area of the furnace between the bottom and the body: (1) gradually and gently decreasing the cross-sectional area from the body (where the cross-sectional area is almost constant); There are three methods: (3) a method in which the cross-sectional area suddenly decreases at the boundary at the bottom of the furnace, and (3) a method in which the two are combined. From the main objective of the present invention, which is to reduce the unit consumption of refractories, either method is acceptable; Method (2) of changing is more advantageous.

(iv) A large value of S _S with respect to S _L means that the slag thickness can be reduced, and the reaction rate can be maintained at a high level when the amount of slag produced is large as in the case of ferrochrome smelting reduction smelting. It is advantageous for Figure 5 shows that the calculated slag thickness at the end of the smelting reduction period (value calculated assuming that no gas exists in the slag) is the reduction rate index of chromium oxide in the slag (Cr% in the slag is 10% to 1%). (expressed as the reciprocal of the time required to reduce the chromium to chromium) and the effect on the achieved chromium value. (In this case, it is assumed that there will be no gas injection into the slag or metal or generation of gas). When the slag thickness exceeds 250 cm, the reduction rate decreases and the achieved chromium percentage increases. This is because there is a limit to the depth to which carbonaceous materials such as coke can penetrate into the stirred molten slag, and beyond that the contact area between the carbonaceous materials and slag decreases, reducing the reduction reaction rate. This is because the contact between the carbonaceous material and the metal is cut off, so when oxygen is blown into the metal, the condition that the metal is saturated with carbon cannot be maintained, resulting in the effect of oxidation of chromium. Therefore, when the slag thickness is 250 cm or more, it is desirable to stop feeding pellets and shift to the second reduction stage. If the amount of slag generated exceeds the above conditions when producing a predetermined amount of metal due to the type of ore or flux conditions, it is desirable to carry out operations such as draining the slag multiple times per tap.

The reason why a top-bottom blowing converter type reaction vessel is more suitable than other furnace types for ferrochrome smelting reduction smelting is as follows.

(i) Without bottom blowing: Due to insufficient stirring, the rate of chromium reduction in the slag is slow. Furthermore, since the metal bath cannot be heated directly, the slag temperature becomes relatively high, which, together with the extension of treatment time, places a significant load on the refractories.

(ii) In the case of only bottom blowing: The number of tuyeres required to blow in a large amount of oxygen increases, making equipment management difficult.
Furthermore, since all of the carbonaceous material must be combusted through oxidation of the metal, if the amount of slag increases, the dissolution of the carbonaceous material into the molten metal will be delayed, making re-oxidation of the molten metal more likely to occur.

(iii) In the case of top-bottom blowing: Top blowing generates heat due to the combustion of carbonaceous material in the slag and the secondary combustion of a part of the CO gas.
It mainly supplies the heat of the reduction reaction, while the bottom blower is used to oxidize the carbon dissolved in the molten metal, and is used to control the molten metal within an appropriate temperature range. There is a fear that bottom-blown O ₂ will re-oxidize the chromium content in the molten metal, but
In the present invention, the molten metal is always kept saturated with carbon (for example, by directly blowing carbonaceous material into the molten metal phase, or by mixing and molding carbonaceous material with ore etc. to make the specific gravity higher than that of the molten slag layer, (For example, by charging from above, the carbonaceous material reaches the molten metal phase, etc.)
By using these in combination as necessary, reduction can be carried out while bottom-blowing acid continues even in the melt reduction stage. This is essential for keeping the slag temperature as low as possible throughout the process and lowering the refractory unit consumption.

Rated molten metal amount (molten metal amount just before tapping) 50t top-bottom blowing converter (1 top blowing lance, nozzle 7mmφ x 7
Holes: 1 hole in the center, 6 holes on the periphery, 25° angle to the lance axis of the 6 holes on the periphery, 4 bottom blowing tuyeres, inner pipe diameter 20 mm
A φ double pipe (propane is flowed into the outer pipe as a protective gas) is used as the reaction vessel, intermediate drainage is performed once, and the molten metal is discharged in 33 heats at a time, leaving 17 tons as hot water, resulting in a semi-continuous process. It was operated in

The shape of the furnace after construction is such that the average cross-sectional area (S _L ) is 30,000 cm ² for a depth of 100 cm at the bottom, and 100 cm from the bottom.
The cross-sectional area (S _S ) of the trunk (straight) of cm or more is
It is ^100000cm2 . After 50 consecutive heat treatments, the average cross-sectional area (S _L ) for the bottom depth up to 100 cm is 41000 cm ² , and the body cross-sectional area S _L for more than 100 cm
is 17300 cm ² and both satisfy the condition of S _S /S _L =2 to 5.

The main raw material, chromium ore, was mixed with coke powder to form pellets, and the coke, an auxiliary raw material, was charged into a rotary kiln as outer coal, and the high-temperature exhaust gas from the smelting reduction furnace was used as a heating source to perform preliminary reduction and preheating. The average composition and temperature of the semi-reduced chromium pellets entering the smelting reduction furnace are as follows. T.Cr36
%, T.Fe18%, chromium reduction rate 66%, iron reduction rate 92%, MgO10%, Al ₂ O ₃ 10%, SiO ₂ 9%, temperature
1020℃. The carbon material is coke with a particle size of 10 mm to 70 mm (C
88%) is passed through the rotary kiln, and the remaining 10% is processed using lime (97% CaO, 20% CaO) depending on the furnace conditions.
~50mm) was directly charged into the smelting reduction furnace.

First reduction stage: Preheated prereduced pellets, coke, and lime are charged into the remaining hot water while blowing oxygen-containing gas from top to bottom. The acid blowing speed is top blowing:
13000Nm ³ /hr, bottom blowing 800Nm ³ ×4/hr. The temperature of the molten metal is measured, and the charging speed of the prereduced pellets is adjusted so that the temperature is controlled between 1580 and 1630°C. Pellet charging is completed in about 60 minutes. At this point, the Cr% in the slag was 3.6%.

Second reduction period: Stop charging the prereduced pellets to the melting reduction furnace, leave the bottom blowing gas as it is, and then
Top blown oxygen 7500Nm ³ /hr, 3000N every 7 minutes
m ³ /hr, ONm ³ /hr, Cr in slag
The final slag composition was changed to CaO19
%, SiO ₂ 20%, MgO 24%, Al ₂ O ₃ 22%, T.Cr:
0.9%, T.Fe 0.7%. This slag is removed as an intermediate slag. The molten metal is left behind, and the above-mentioned first and second
Repeat the reduction period, then continue with the slag,
Tap out 2/3 of the molten metal. After that, the same cycle is repeated. The components of the tapped metal are as follows. Cr: 53%, Fe: 37%, C: 8.5%, Si:
0.7%, S: 0.015%, P: 0.035%.

As described above, the present invention solves the problems of the smelting reduction method that conventionally produced ferrochrome in an electric furnace without using electricity, by combining special operation of the pre-reducing furnace, and by changing the shape of the smelting reduction furnace. This problem was solved by specifying the operating conditions and made it possible to manufacture high-chromium alloys at low cost, which has a large economic effect.

[Brief explanation of the drawing]

Figure 1 shows one of the facilities used to carry out the present invention.
An explanatory diagram showing an example. Figure 2 is a diagram showing the influence of the preliminary reduction rate of chromium pellets on the reduction rate of chromium oxide in slag. Figure 3 is a diagram showing the influence of the preliminary reduction rate of chromium pellets on the refractory erosion index. Figure 4 is a diagram showing the influence of the internal shape of a top-bottom blowing converter on the chromium reduction rate index and refractory consumption index, and Figure 5 is a diagram showing the influence of the inner shape of the top-bottom blowing converter on the chromium reduction rate index and refractory consumption index. FIG. 3 is a diagram showing the influence of the chromium oxide (calculated on the assumption that no gas exists in the slag) on the reduction rate of chromium oxide in the slag and the attained chromium value of the slag.

Claims (1)

[Claims] 1. A high chromium alloy smelting reduction smelting method for producing a high chromium alloy from chromium ore as a raw material using a preliminary reduction furnace and a top-bottom blowing converter type smelting reduction furnace,
After reducing more than 90% of the iron oxide in the chromium ore using external charcoal in a pre-reduction furnace, this pre-reduced chromium ore (chromium raw material) is reduced to the average cross-sectional area of the bottom (non-straight part). In the smelting stage, the pre-reduced chromium ore (chromium material)
A method for smelting and reducing high chromium alloys, comprising a smelting stage in which the supply of chromium is stopped and the blowing acid and bath are stirred.