JPS58485B2 - Start-up control method for shaft furnace for continuous sintering and reduction of steel powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a start-up control method for a shaft furnace for sintering and reducing copper powder.

Steel powder, especially alloyed steel powder, is often prepared mainly by the so-called water atomization method, but it cannot be applied to commercialization by powder metallurgy as it is, and it is not possible to apply it as is to commercialization by powder metallurgy. A material for powder metallurgy can only be obtained through sintering reduction, which corresponds to the final reduction process performed in the process.

However, with steel powder alloyed mainly with strongly oxidizing elements such as Mn and Cr, the surface of the powder particles is easily oxidized by the water atomization method, forming a hard-to-reducible oxide film, so the conventional finishing reduction process cannot be used. It was difficult to reduce the amount.

Therefore, the need to enable a commensurate sintering reduction of the atomized steel powder is now becoming an urgent need.

In this regard, the inventor previously disclosed in Japanese Patent Application No. 51-26708 that in order to overcome the above problem, the above-mentioned stiff steel powder was preloaded with carbon, and the steel powder was preheated and sintered in a reduced pressure atmosphere. We have already disclosed the basic development results of the advantageous suitability of sintering self-reduction by high-frequency induction heating in the field of sintering, together with the particular suitability of vertical or shaft furnaces as an embodiment thereof.

This invention focuses on the problems inherent in sinter reduction in shaft furnaces, among the many new difficulties encountered by the inventors during subsequent research and development regarding the actual operation of shaft furnaces. An effective solution for the start-of-work control method is proposed in detail below.

The raw steel powder used for sintering reduction in the shaft furnace includes:
For example, a steel powder pre-alloyed with carbon prepared by the atomization method may be used as is, or the amount of carbon contained in the steel powder may be reduced by reducing the pressure of the raw material steel powder (in this invention, it is simply referred to as "reduced pressure" including so-called vacuum, and the degree of such reduction is simply referred to as "reduced pressure"). A mixture of carbonaceous materials (e.g., graphite, charcoal, coke, etc.) powder in an amount necessary to compensate for the reduction reaction of steel powder particles in an atmosphere (referred to as a degree of fluidity) is used. Rather, a powder-like form is used for convenience in processing and handling, and the synchronization between the reaction process of sinter reduction by high-frequency induction heating and the progress of the preheating sintering process before proceeding to this process is important. The accumulation level of the raw material powder in the core tube of the preheating furnace due to the lowering of the guide dummy installed in the shaft furnace in a raised manner to support the first cut and charged raw material steel powder for preheating. It is necessary to smoothly and reliably supply the raw material powder to compensate for the decrease.

The specific purpose of the present invention is to provide a system that is particularly advantageous for smooth start-up control of this type of shaft furnace by special devising of the descending movement of the leading dummy during the initial preheating stage and the accompanying replenishment of raw material powder. It is about realizing solutions.

In such a preheating stage, the reduction reaction of FeO-based oxides mainly proceeds advantageously in the vicinity of 1000°C, which corresponds to that temperature range, so preheating in a reduced pressure atmosphere is performed in a shaft furnace. It is preferable to apply this from the beginning of the operation, but even in this case, the degree of vacuum applied near the bottom of the preheating furnace is made slightly weaker than that on the cutting side of the raw material powder, so that the powder is charged into the preheating furnace. It is possible to advantageously prevent the vertical pressure difference of the raw steel powder from becoming a trigger for promoting the flowing down of the raw powder bed.

An example of a shaft furnace used for carrying out the present invention is shown in FIG.

In the figure, 1 indicates a preheating furnace, and 2 indicates a high frequency furnace, and these are arranged vertically in that order as shown in the figure.

The preheating furnace 1 is configured to burn C gas and other fuels between a core tube 3 made of a heat-resistant steel pipe that is slightly tapered downward and a refractory wall 4 that surrounds the core tube 3. Alternatively, it has a combustion chamber or a heating chamber 5 that performs external heating by resistance heating, and 6 is an iron skin.

Note that this figure shows an example of a C gas furnace, but the burner and exhaust gas flue are not shown.

The high frequency furnace 2 has an induction heating coil 7 connected to the furnace core tube 3 of the preheating furnace 1 via a sliding flange joint 8, and arranged around a one-turn secondary coil 9. An insulating inner cylinder 10 made of alumina porcelain is installed inside.

A two-stage raw material hopper 12 is placed at the top of the preheating furnace 1 via a cutting device 11 .

The raw material hopper 12 stores raw material steel powder, and in the illustrated example, it is guided into a fixed volume of a slide feeder 13, and an elevating twin 14 detects the charging level of the raw material steel powder in the furnace core tube 3 in a timely manner. By operating the slide feeder 13 according to the stroke of the feeder, the raw steel powder is intermittently cut and charged into the furnace core tube 3.

The first charging of this raw steel powder is carried out in the preheating furnace 1 into the furnace core tube 3.
A rising type leading dummy 15 is provided to allow the deposition inside.

The leading dummy 15 is made up of a dummy head 16 that fits relatively loosely near the lower end of the furnace core tube 3, and a steel wool packing 17 superimposed and fixed on the dummy head 16, which is held on a long elevating stem 18.

Although the elevating stem 18 is not shown, it is possible to control the lowering at a slow speed of several mm/min, for example, by means of a rope.

In the figure, reference numeral 19 denotes a pinch roll 20 that supports the weight of the sintered reduction cake obtained according to the present invention (hereinafter referred to as ``cake'') from the leading dummy 15 in accordance with its formation.
1 is a cake coarse crushing cutter, 21 is a fixed discharge chute,
22 is a packet, 23 is an outlet door, and 24 is a cake cooling chamber.

In the operation of the shaft furnace according to the present invention, the slide feeder 13 is first operated in the illustrated raised position of the leading dummy 15, and the raw steel powder in the raw material hopper 12 is transferred to the preheating furnace.
The first step is to cut out the material into the furnace core tube 3 and charge it intermittently.
Starting from the start, during the restoring process of the slide feeder 13, the tween 14 performs an empty stroke to repeat the cutting and charging operation of the slide feeder 13, and as a result, the raw steel powder is transferred to the upper end of the leading dummy 15 in the core tube 3. When the steel wool 17 is filled and a columnar deposit is formed, the tween 14 detects this during the stroke, and as a result, the slide feeder 13 temporarily stops while the combustion chamber in the preheating furnace 1 Gas ignition is performed, and due to the external heating, the columnar deposit held on the leading dummy 15 undergoes a sintering reaction radially inward from its outer periphery, and as a result, the columnar deposit has a cylindrical shell shape. Growth occurs such that the sintered zone becomes thicker.

This preheating temperature is generally in the range of 780 to 1200°C, and is appropriately selected depending on the composition of the raw steel powder, the scale of the equipment, and other conditions. Sintering is continued for a period of time necessary to support the weight of the raw material steel powder further deposited inside and on top, and to eliminate the fear of raw powder flowing down due to crumbling.

At this time, according to a test in which the outer diameter of the columnar deposit was set to 160 mm, the wall thickness of the cylindrical shell-shaped sintered region was 30 mm.Therefore, the inner unsintered part had a sufficient outer diameter of about 100 mm to start descending. was gotten.

The columnar deposit containing this sintered region will be particularly referred to as a P cake.

Waiting for the above P cake to be baked, the temperature of the preheating furnace 1 is lowered to about 500°C when the preheating temperature is set at 1100°C.

As the temperature decreases, the thermal contraction that occurs in the heat-resistant steel furnace tube 3 causes radial tightening and compaction of the P cake, and when the preheating furnace is then heated again, only the furnace core tube 3 expands in advance. As a result, a slight gap is created between the P cake and the inner wall of the furnace core tube 3, and the smooth core descent of the P cake is prepared.

The P cake that has started to fall in this way is subsequently heated by high frequency induction, but at this time it is also necessary to heat the P cake under gradual increases in the high frequency input as described below.

That is, after the above temperature drop, the preheating temperature is 11
The preheating furnace temperature is raised again to 00°C, and the dummy head 16 of the leading dummy 15 is heated in the high frequency furnace 2 while growing the cylindrical grain sintered area.
During this time, the raw material steel powder charged subsequently is preheated, and a cylindrical sintered region is successively grown in the vertical direction.

In addition, as a method for maintaining the deposition level of the raw material powder in the preheating tube at a fixed position even while the raw material powder is descending, Tween-14, which is repeatedly raised and lowered intermittently by a timer, keeps its lower end at the deposition level of the raw material powder. Before contacting the slide feeder 13, a signal is sent to the slide feeder 13 by pressing the limit switch (not shown) for setting the lower limit position and turning it on.
This is done by adding additional powder.

After that, the Tween-14 descends again, and if the lower limit switch is turned on again, more powder is added, and if the lower limit switch is turned on again, the lower limit of the Tween-14 When the Tween-14 comes into contact with the deposition level of the raw material powder, the Tween-14 rises and stops at the upper limit position after the timer expires, and no additional powder is added.

By the above method, the deposition level of the raw material powder is maintained at a fixed position.

Thus, P sequentially extends downward inside the shaft furnace.
Inside the cake, a downwardly convex parabolic sintered-unsintered interface, ie, a sintering front, is formed with the bottom point at the upper end of the primary coil 7 in the high-frequency furnace 2.

After this sintering front reaches just above the heating zone of the high frequency furnace 2, the high frequency input is gradually increased to transform the P cake into the aforementioned cake.

This gradual increase in high-frequency input is based on the example above, i.e. the outer diameter 16
For a cake of 0 mm, good results were obtained using a step method in which the number of kilowatts was reached at 10 minute intervals in 2KW increments, and it took a little more than one hour.

After that, the P cake generated in the preheating furnace 1 is successively transformed into a
reaching the bell is easily detected by the stem 18 of the leading dummy 15, for example by actuating a detection contact, thereby allowing the load on the leading dummy 15 to be supported by the pinch roll 19, where Subsequently, the leading dummy 15 is rapidly lowered to the evacuation position, and during this time another detection contact is operated, thereby moving the chute 2 to a position directly below the pinch roll 19 and cutter 20, from the previous evacuation position. The coarsely crushed cakes can be guided into the packet 22 one after another by the cutter 20, which is advanced from the center and at the same time starts repeated driving at an appropriate time.

Although I did not mention it to avoid complicating the explanation, the reaction of 2MO+C→2M1002 mainly occurs in the relatively low temperature area of the preheating furnace 1, and from the high temperature area of the preheating furnace 1 to the high frequency furnace 2.
In the heating zone, the reaction MO+C→M+CO proceeds using the carbon contained in the raw steel powder or the carbonaceous material added thereto as a reducing agent.

In order to eliminate such generated gas and promote the above-mentioned reduction reaction, it is necessary to provide vacuum system piping to each part of the shaft furnace to constantly exhaust the inside of the furnace.

That is, as shown in the illustrated example, for example, a mechanical booster MB1 and a rotary vacuum pump RP are connected through a valve A at the bottom of a shaft furnace, and a cutting device 11 is built in between the valve A and the mechanical booster MB1. A valve B is provided between the furnace top t and the furnace bottom, and a mechanical booster MB2 and a rotary vacuum pump RP2 are further connected to the furnace top t via the valve valve. Piping is connected between the booster MB2 and the valve via the valve E to the furnace bottom.

This vacuum system performs the following valve operations at the time of starting and steady operation of the above-mentioned shaft furnace.

By opening the valve C halfway during the start operation, the degree of depressurization near the lower end of the core tube 3 of the preheating furnace 1 can be determined from the hollow part M in the furnace bottom or the sliding flange joint 8.
By making the pressure slightly weaker than that at the top of the furnace t, the difference in pressure between the upper and lower sides of the furnace core tube 3 can sometimes cause the columnar pile of raw steel powder to be blown away before the P cake is formed, causing the raw powder to flow down. This is to complete the situation.

In steady operation 1, the vacuum systems for the furnace bottom and furnace top T are made independent, and in steady operation 2, it is the furnace bottom that further strengthens the degree of depressurization at the furnace bottom. This is to avoid the risk of re-oxidation of the cake surface layer due to CO2 during the reaction.

In this way, it is possible to advantageously lead to reliable continuous production of P and 1 cake, but in the preheating furnace 1, the preliminary reduction of FeO is mainly carried out, and also in the high frequency furnace heating zone 12.
In the 00 to 1400°C range, the subsequent reductions of MnO, Cr2O3, etc. proceed favorably, and in order to continue such reduction reactions, it is important to select the rate of descent of the cake and ultimately the P cake. The reason for this is the importance of selecting a rate of descent such that the sintering front has already disappeared just above the heating zone of the high frequency furnace, based on the results of the inventors' numerous experiments and repeated trial and error. I was scowled.

This is because when the sintering front enters the high-frequency heating zone, the power of the high-frequency coil is lost due to the sintering reaction in the unsintered region, making the reduction reaction incomplete.

As described above, according to the present invention, the raw material as a problem peculiar to a shaft furnace, which is a particular problem when initially charging raw steel powder into a shaft furnace for efficient sintering reduction of steel powder. Sintering and reduction reactions can be advantageously carried out under the control of the leading clumps of the powder, and moreover, simple and reliable furnace operation can be realized.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view of a shaft furnace showing how the present invention can be implemented. 1...Preheating furnace, 2...High frequency furnace, 3.
... Core tube, 15 ... Leading dummy.

Claims (1)

[Claims] 1. From the bottom of the shaft furnace in which the preheating furnace and high frequency furnace are arranged vertically, the material is raised and lowered while passing through the high frequency furnace and reaching the core tube of the preheating furnace, and is sequentially cut out from the top end at the bottom end of the core tube. A guide dummy sealing the raw material powder to be charged with carbon on the inner periphery of the preheating furnace tube is used to prevent thickening of the cylindrical sintered region of the charged raw material powder in a reduced pressure atmosphere by external heating of the preheating furnace tube. The sintering process is performed by high-frequency induction heating under reduced pressure on the sintering area directly below the sintering front, which has a parabolic shape at the interface of the unsintered powder in the cylindrical sintered area. A step in which the leading dummy is continuously lowered in accordance with the progress of the reduction reaction occurring in individual particles, and a drop in the deposition level of the raw material powder in the core tube of the preheating furnace due to the lowering of the leading dummy is detected through both stages. A shaft furnace for continuous sintering and reduction of copper powder, characterized in that the replenishment and cutting and charging of raw material powder to maintain the raw material powder at a constant level is combined with a process that is performed sequentially according to the decrease in the level. Starting control method.