JP7593594B2 - Granulation Method - Google Patents

Description

The present invention relates to a granulation method using raw materials containing powder dust.

It has been known that powder dust is generated from electric furnaces used for melting/oxidizing refining stainless steel. This powder dust is a fine powder with a particle size of 10 μm or less, so there is no application for it as it is. In addition, it may contain elements such as Cr that are harmful to the environment. For this reason, much of this dust is treated as industrial waste, and it has been pointed out that it cannot be easily disposed of in landfills, etc., and that disposal costs are required.

Furthermore, since the Ni, Cr, Fe, etc. contained in the powder dust are valuable metals, it is also important to find a use that can maximize the intrinsic value of the elements from the viewpoint of effective utilization of metal resources. However, in order to recover such valuable metals, it is preferable to granulate the powder dust into a granulated material that is easy to use.
In order to solve the above problems, it has already been considered to collect the powder dust, granulate it into a larger particle size, and use it for new purposes other than disposal. For granulating such powder dust, the first thing that comes to mind is the use of a granulation method using a conventional pan pelletizer or drum pelletizer.

However, the granulation method using a pan pelletizer or drum pelletizer requires many operational factors, such as the angle of the granulator (which affects the growth of the granules), the position and amount of water added, and scraping off the semi-granules that have adhered to unintended parts of the device. As a result, only experienced workers can perform satisfactory granulation, and therefore the method is not suitable for obtaining granules from powder dust.
Therefore, techniques for granulation without using such general pan pelletizers or drum pelletizers have been developed as described in Patent Documents 1 to 3.

For example, Patent Document 1 discloses a granulation method for sintering raw materials that uses a sintering raw material mainly composed of fine powder (raw material containing 60% by mass or more of particles with a particle size of under 500 μm), and that can granulate granules having a target particle size distribution at a higher yield than conventional methods. This granulation method of Patent Document 1 is a method that can perform two processes, kneading and granulation, in one device, and is characterized in that, when the inner diameter of the horizontal container is D (m), the peripheral speed of the stirring blade is u (m/s), and the gravitational acceleration of the sintering raw material is G (m/s ² ), the stirring acceleration of the stirring blade 2×u2/D (m/s ² ) is set within a range of 2 G (m/s ² ) to 10 G (m/s ² ).

Furthermore, Patent Document 2 discloses a method for granulating a sintering raw material mainly composed of fine powder (a raw material containing 60 mass % or more of particles with a particle size exceeding 0 μm and 250 μm or less), which can produce granulated products having a target particle size distribution at a higher yield than conventional methods.
Furthermore, Patent Document 3 discloses a recycling method in which zinc-containing converter dust generated in the converter process during steel production is separated into dust with a low zinc content (dust with an average particle size in the range of 8 μm to 25 μm and a zinc content of 1 mass % or less) and dust with a high zinc content, and then efficiently recycled to a blast furnace, converter, or electric furnace. The recycling method of Patent Document 3 is said to make it possible to reduce the cost required for dezincing treatment.

JP 2009-242939 A JP 2009-287122 A Republished WO08/032638

However, granules produced using a pan pelletizer or drum pelletizer have the following problems.
That is, the granulated material produced using a pan pelletizer or drum pelletizer contains a large amount of moisture. Since the granulated material containing a large amount of moisture is brittle, it is necessary to remove the moisture and increase the strength of the granulated material in order to use the granulated material. Specifically, it is essential to remove the moisture by drying or the like to improve the strength of the granulated material so that the strength of the granulated material can stably satisfy 3.0 kg/ ^cm2 . In other words, granulating machines such as pan pelletizers and drum pelletizers need to secure a certain number of storage (curing) days until the moisture is removed and the strength is improved, and the productivity is reduced by the amount of storage required for removing the moisture.

In addition, since the working population is expected to decrease in the future, it is desirable to be able to granulate without relying on the skills of experienced workers. In this respect, pan pelletizers and drum pelletizers that rely on the skills of experienced workers are not preferable.
On the other hand, the granulation method of Patent Document 1, which is a technology that does not use a general pan pelletizer or drum pelletizer, is said to be able to improve the yield of granulated material having a targeted particle size distribution by growing the granulated material until it reaches a targeted particle size, and therefore specifies the stirring acceleration of the stirring blades, etc. However, since no consideration is given to the strength of the granulated material, it is not possible to realize high granulated material strength or good productivity.

Furthermore, even in the granulation method of Patent Document 2, the rotation speed of the granulator is low at about 20 rpm, and it is hard to imagine that granulated material with excellent strength can be obtained with such low-speed rotation.
Furthermore, the recycling method of Patent Document 3 also uses fine dust having an average particle size of 8 μm or more and 25 μm or less as a raw material, but does not consider the rotation speed of the granulator, and at least it cannot be considered that the strength of the granulated material is sufficient.

In other words, none of the methods in Patent Documents 1 to 3 can produce strong granules. Therefore, there has been a demand for establishing a technology for producing granules having a predetermined strength, i.e., granules having a strength of 3.0 kg/cm2 or ^more that cannot be crushed with fingers, from fine dust generated in a stainless steel smelting furnace or the like, using a semi-automatic device with few operating factors.
The present invention has been made in consideration of the above-mentioned problems, and has as its object to provide a granulation method capable of granulating fine dust with a strength of 3.0 kg/cm2 or ^more with a good productivity, so that the proportion of granules with a particle size of 4.75 mm or more is 95% or more.

In order to solve the above problems, the delivery body of the present invention employs the following technical measures.
That is, the granulation method of the present invention is characterized in that it uses powder dust having a particle size of 20 μm or less generated from an electric furnace for melting stainless steel or an electric furnace for oxidatively refining stainless steel as a raw material, and feeds water and the raw material into a kneading machine so that the water content is more than 10 mass% and not more than 12 mass% of the total amount, kneads the raw material, feeds the kneaded raw material into a granulator equipped with a granulation chamber having a rotating bed portion rotating at 60 to 90 revolutions per minute and a fixed side wall portion, and granulates the raw material while rotating the rotating bed portion in the granulation chamber for 2 minutes or more, thereby obtaining a granulated product in which particles having a particle size of 4.75 mm or more account for 95% or more of the total particles .

Preferably , the powdered raw material contains FeO: 0.1 to 10 mass%, Fe ₂ O ₃ : 25 to 45 mass%, CaO: 2 to 15 mass%, SiO ₂ : 5 to 15 mass%, Al ₂ O ₃ : 0.1 to 15 mass%, MgO: 0.1 to 5 mass%, MnO: 2 to 15 mass%, Cr ₂ O ₃ : 10 to 20 mass%, with the remainder being iron and unavoidable impurities, and has a particle size of 20 μm or less.

It is preferable to use powdered raw materials whose primary particle diameters are all 10 μm or less.

According to the granulation method of the present invention, it is possible to granulate fine dust with a strength of 3.0 kg/cm2 or ^more with a ratio of particles having a particle size of 4.75 mm or more to 95% or more with good productivity. Note that the raw material for the granulation material in the present invention is not limited to powder dust from the stainless steel manufacturing process, but can be widely applied to fine dust with a particle size of 20 μm or less, such as converter dust.

FIG. 1 is a diagram showing a granulation facility used in the granulation method of the present invention. FIG. 2 is a cross-sectional view showing the granulator of the present embodiment. 1 is a graph showing the relationship between the rotation speed of the rotating bed of the granulator and the proportion of granulated material having a particle size of 4.75 mm or more relative to the total granulated material. 1 is a graph showing the relationship between the sieve opening and the proportion of granulated material captured on the sieve depending on the opening. FIG. 2 is a schematic diagram showing the forces acting on a granulated material during granulation. FIG. 2 is a schematic diagram showing the trajectory of granulated material moving in the granulation chamber during granulation.

Hereinafter, an embodiment of a granulation method according to the present invention will be described in detail with reference to the drawings.
The granulation method of the present embodiment is intended to obtain granules having a strength of 3.0 kg/cm ² or more from powdered raw material M having a particle size of 20 μm or less.
The powdered raw material M described above is generally called powder dust generated from an electric furnace for melting/oxidation refining stainless steel. This powder dust contains valuable metals such as Ni, Cr, Fe, etc., and in order to make effective use of metal resources, it is preferable to adjust the powder dust from its dust form, which is difficult to use, to a granulated material with a larger particle size. In the present invention, the composition of the powdered raw material M is not particularly limited, but for example, the powdered raw material may contain FeO: 0.1-10 mass%, Fe ₂ O ₃ : 25-45 mass%, CaO: 2-15 mass%, SiO ₂ : 5-15 mass%, Al ₂ O ₃ : 0.1-15 mass%, MgO: 0.1-5 mass%, MnO: 2-15 mass%, Cr ₂ O ₃ : 10-20 mass%, and the balance is iron and unavoidable impurities, and has a particle size of 20 μm or less. More preferably, the primary particle size of all particles is 10 μm or less. In addition, it is preferable to use the powdered raw material M described above with a secondary particle size of 0.850 mm or less.

Fine powder (powder dust) with a particle size of 20 μm or less is too small to be used as is, and since it contains elements (harmful elements) such as Cr as described above, it may not be easy to dispose of it in landfills as is.
In the present invention, the powder dust is granulated into a granulated product having a larger particle size than dust and is easier to use, thereby expanding the uses of the powder dust, which was previously difficult to reuse, and from the viewpoint of effective resource utilization, making it possible to make maximum use of the intrinsic value of elements (valuable metals) such as Ni, Cr, Fe, etc. contained in the powder dust as metals or oxides.

Specifically, the granulation method of the present invention involves feeding water to a kneader 2 so that the total amount is greater than 10 mass% and less than or equal to 12 mass%, and then feeding the mixed raw material M into a granulator 3 equipped with a granulation chamber 6 having a rotating bed 4 that rotates at 50 to 150 revolutions per minute and a fixed side wall 5, and granulating the raw material M in the granulation chamber 6 while rotating the rotating bed 4 for at least 2 minutes to obtain a granulated product. It is preferable to granulate the raw material M in the granulation chamber 6 while rotating the rotating bed 4 at 60 to 110 revolutions per minute.

Next, the granulation equipment 1 used in the granulation method of the present invention and the granulation conditions will be described in detail.
As shown in FIG. 2, the granulation equipment 1 of this embodiment is composed of a raw material supplying machine 7, a kneading machine 2, and a granulator 3.
The above-mentioned raw material supplying machine 7 includes a hopper 8 formed in a tapered shape so as to widen toward the top, and a shutter 9 provided at the bottom end of the hopper 8. The hopper 8 is formed in a tapered shape so as to widen toward the top, so that the raw material M can be collected without spilling. The raw material M supplied from the top of the hopper 8 is sent to the shutter 9 provided at the bottom of the hopper 8. The shutter 9 is capable of blocking the supplied raw material M at regular intervals, so that the raw material M can be supplied at a fixed amount. The material that passes through the shutter 9 is sent to the kneader 2 (one end side of the kneader 2).

The kneader 2 is a continuous kneader 2 having two kneading screws with their axes oriented horizontally.
The raw material M supplied between the two kneading screws is subjected to a shearing force due to kneading, thereby kneading the raw material M. The kneader 2 is configured to supply the raw material M from one end connected to a hopper 8, knead the supplied raw material M while sending it to the other end, and discharge the kneaded raw material M from the other end connected to a granulator 3.

In addition, the other end of the kneader 2 in this embodiment is positioned at the same height as the upper end of the granulator 3 or higher than the granulator 3, making it possible to supply the kneaded raw material M from above the granulator 3.
2 and 3, the granulator 3 includes a granulation chamber 6 for granulating the raw material M mixed in the mixer 2. In this embodiment, the granulation chamber 6 is formed in a substantially cylindrical shape with its axis oriented in the vertical direction. The side wall 5 of the granulation chamber 6 is fixed, whereas the rotating bed 4 is rotatable, so that the raw material M supplied thereto can be granulated while being rolled.

Specifically, the granulator 3 of this embodiment is equipped with a cylindrical granulation chamber 6 with a bottom that opens upward. The side wall 5 of the granulation chamber 6 is formed as a vertical wall that stands up in the vertical direction, and the bottom wall 10 of the granulation chamber 6 is formed as a horizontal wall that extends in the horizontal direction. A through hole 11 that penetrates the bottom wall 10 in the vertical direction is formed in the center of the bottom wall 10. It is preferable that the side wall 5 formed in the granulation chamber 6 has a circular shape when observed from above the device, as shown in Figures 6 and 7, but this is not limited to this as long as the raw material M is processed as described below. For example, a polygonal shape is also acceptable.

A rotating bed 4 is provided above the bottom wall 10 of the granulation chamber 6. The rotating bed 4 is a separate member from the bottom wall 10 constituting the granulation chamber 6, and is formed in a disk shape that can be fitted into the granulation chamber 6 from above. The rotating bed 4 is provided above the bottom wall 10 of the granulation chamber 6, and is rotatable relative to the fixed bottom wall 10.
A shaft 12 that protrudes downward is provided on the underside of the center of the rotating bed 4. This shaft 12 is rotatable integrally with the rotating bed 4, and is inserted from above into the through hole 11 of the bottom wall 10. The lower end of the shaft 12 is connected to a drive motor 13, and the rotational driving force of the drive motor 13 can be transmitted via a power transmission mechanism 14 such as a worm gear or a bevel gear.

Therefore, when the drive motor 13 is driven, the rotational driving force of the drive motor 13 is transmitted to the shaft 12 via the power transmission mechanism 14, and the disk-shaped rotating bed 4 rotates around an axis facing in the vertical direction. In other words, the above-mentioned granulation chamber 6 is structured so that the rotating bed 4 rotates around an axis facing in the vertical direction relative to the side wall 5 of the granulation chamber 6, which is fixed and does not rotate, and the raw material M supplied to the granulation chamber 6 can be granulated while rolling on the rotating rotating bed 4.

A discharge port 15 is formed on the underside of the side wall 5 of the granulation chamber 6 to remove the granulated material from the chamber after granulation has been completed. The discharge port 15 is formed to penetrate the side wall 5 of the granulation chamber 6 from inside to outside, and is formed at a position where the lower edge of the opening is flush (same height) with the upper surface of the rotating bed 4, so that the granulated material that rolls on the rotating rotating bed 4 and rolls to the outer periphery due to centrifugal force can be removed to the outside.

The outlet 15 is provided with a granulation size adjusting plate 16 for adjusting the size of the granulation material taken out of the chamber from the outlet 15. The granulation size adjusting plate 16 is a plate member capable of closing a lower portion of the opening of the outlet 15.
That is, the supplied raw material M is processed in the granulation chamber 6 formed in a cylindrical shape, and remains in the granulation chamber 6 for a predetermined processing time. In the granulation chamber 6, the particle size of the granulated material rolling on the surface of the rotating bed 4 increases as the rolling time increases, and the granulated material gradually grows into a larger size. Eventually, the granulated material that has reached a target particle size or larger is automatically discharged by climbing over the granulation size adjustment plate 16. For example, if the above-mentioned granulation size adjustment plate 16 is present, granulated material with a small particle size cannot climb over the granulation size adjustment plate 16 and is not taken out of the chamber. However, if the particle size has grown to a large size, the granulated material can climb over the granulation size adjustment plate 16, and only granulated material that has grown to a certain size is taken out of the chamber.

That is, the granulation size adjusting plate 16 functions as a restricting plate (baffle plate) that restricts the discharge of granulation material that does not meet a predetermined particle size, and has a function of adjusting the size of the discharged granulation material by allowing only granulation material with a predetermined particle size or more to be taken out. Therefore, when granulation material with a large particle size is to be granulated, it is preferable to lengthen the processing time of residence in the granulation chamber 6 and to increase the vertical width of the granulation size adjusting plate 16, in other words, the height of the granulation size adjusting plate 16. On the other hand, when granulation material with a small particle size is to be granulated, it is preferable to shorten the processing time of residence in the granulation chamber 6 and to decrease the vertical width of the granulation size adjusting plate 16, in other words, the height of the granulation size adjusting plate 16.

When granulation is performed using the granulation equipment 1 having the above-mentioned configuration, the strength of the granulated material must be strong enough to not be crushed even when pinched with fingers, specifically, a strength of 3.0 kg/ ^cm2 or more. If the strength of 3.0 kg/cm2 or ^more is not satisfied, the granulated material will easily crumble and become difficult to use. In addition, in order to further increase the strength of the granulated material, additional curing and drying will be required, which will increase the cost of granulation and reduce productivity.

On the other hand, when it is used as a granulated product, it is necessary to granulate the granulated product to a size that is easy to use. In this regard, in the granulation method of the present invention, it is preferable to granulate the granulated product to a size (particle size distribution) such that the ratio of particles having a particle size of 4.75 mm or more to the total particles of the granulated product is 95% or more.
Furthermore, when producing granules having the above-mentioned strength and size, it is also necessary to perform granulation with good productivity.

Therefore, in order to satisfy all of the above-mentioned strength, particle size distribution, and productivity of the granulated material, the granulation method of the present invention rotates the rotating bed 4 at a rotation speed of 50 to 150 revolutions per minute, preferably 60 to 110 revolutions per minute, for 2 minutes or more to perform granulation. Note that this processing time refers to the time from when the raw material M such as powder dust is completely fed from the hopper 8 into the granulation equipment 1 until the granulated material is discharged from the discharge port 15.

When granulation is carried out at the above-mentioned rotation speed and processing time, the granulated material is compacted as it rolls on the surface of the rotating bed 4, and at the same time, moisture is removed from the granulated material, improving its strength, making it possible to efficiently produce granulated material with a strength of 3.0 kg/ ^cm2 or more.
Moreover, the longer the processing time for granulation in the granulation chamber 6, the larger the particle size of the granulated material becomes. When granulation is performed for 2 minutes, the particles grow until the majority of the particles have a particle size of 4.75 mm or more. Therefore, by providing a granulation size adjusting plate 16 with a height of, for example, 50 mm, and adjusting the size of the granulated material to be taken out, it is possible to obtain granulated material with a particle size distribution in which the proportion of particles having a particle size of 4.75 mm or more is 95% or more.

By using the above-mentioned granulation equipment 1 and rotating the rotating bed 4 for 2 minutes or more at a rotation speed of 50 to 150 rpm, preferably 60 to 110 rpm, it is possible to obtain granulated products with excellent strength, particle size distribution, and productivity. As a result, even if the powdery raw material M has a particle size of 20 μm or less, such as fine powder dust, it is possible to granulate the granulated products with a strength of 3.0 kg/cm2 ^{or more} and a particle size distribution in which 95% or more of the granulated products have a particle size of 4.75 mm or more.

In addition, with the above-mentioned granulation method, moisture is sufficiently removed from the granulated material by granulation, so storage (curing) and drying to remove moisture, as in conventional granulation methods, are not necessary, and the number of days required for these processes can be omitted from the production process, dramatically increasing the productivity of granulation.

Next, the effects of the granulation method of the present invention will be described in detail using comparative examples and examples. In the examples and comparative examples, powdered raw materials with a particle size of 20 μm or less were granulated using the granulator 3 of the present invention, a pan pelletizer, and a drum pelletizer under the conditions shown in Tables 1 and 2, and the strength, moisture content, and particle size distribution of the granulated product were measured.

The strength is the crushing strength measured using a sample compression/destruction type crushing strength tester, and is judged based on whether it is "3.0 kg/ ^cm2 or more," which is a strength that cannot be crushed with fingers. As described above, if the granulated material has a strength of "3. kg/ ^cm2 or more," it can be judged that "the handleability of the pellets can be improved."
Furthermore, if the above-mentioned strength can be achieved immediately after granulation, the number of days of storage (curing) required after granulation until moisture is removed and strength is improved can be omitted, which can be considered to be "expected to reduce manufacturing costs and improve productivity."

The strength was measured immediately after granulation, 3 days after granulation, and 7 days after granulation.
The moisture content was measured by measuring the moisture content of the granules immediately after granulation using an infrared heating type dry mass meter.
Furthermore, the particle size distribution is "the percentage by weight of granulated material with a particle size of 4.75 mm or more to the total particles."

The results of the above-mentioned examples and comparative examples are shown in Table 1.

Looking at the crushing strengths of Comparative Example 1 and Comparative Example 2 in Table 1, it can be seen that Comparative Example 1, in which a pan pelletizer was used for the granulator 3, and Comparative Example 2, in which a drum pelletizer was used, had very low crushing strengths of 0.5 kg/ ^cm2 and 1 kg/ ^cm2 , and were more likely to be crushed than the crushing strengths of 4.9 kg/ ^cm2 to 7.2 kg/ ^cm2 in the Examples. This is thought to be because in pan pelletizers and drum pelletizers that do not have a stationary side wall portion 5 and a rotating rotating bed portion 4 in the granulation chamber 6, the granulated material does not roll, or even if it does, the rolling speed is slow. In other words, Comparative Example 1 and Comparative Example 2, in which the rolling is insufficient, do not impart large distortion to the granulated material, and the granulated material is also weak, so that the target crushing strength of "≧3 kg/ ^cm2 " cannot be met. In contrast, the granulator 3 of the embodiment, in which the bottom surface rotates relative to the stationary side wall portion 5, has a large difference in relative speed between the side wall portion 5 and the bottom surface, and the granulated material can be rolled strongly. As a result, it is possible to impart a large strain to the granulated material, and the granulated material becomes stronger, and it is judged that the target crushing strength of "≧3 kg/ ^cm2 " can be satisfied.

In addition, in Comparative Examples 1 and 2, workers who are skilled in actually operating the equipment were in charge of the work. Such work includes the addition of raw material M, the supply of moisture (adding water in a way that sprinkles it evenly), and the supply of new raw material M (adding raw material M in a way that sprinkles it evenly in the same way as water), and the finished quality of the granulated product is good due to the skilled skills of the workers.

In other words, in Comparative Examples 1 and 2, because the work was performed by workers with skilled techniques, the percentage of granulated material with a particle size of 4.75 mm or more, which is the target particle size, i.e., the yield, was high at 99.1% and 99.3%.
However, the inventors have confirmed that when the work is performed by a worker with ordinary skills or a worker with a low level of proficiency, the proportion of particles with a particle size of 4.75 mm or more will be lower than the above-mentioned value, and it will be difficult to achieve a stable yield. Therefore, it can be determined that the result in the embodiment, where "the proportion of particles with a particle size of 4.75 mm or more will be stably 95% or more," is highly likely to be unsatisfactory.

Next, using the granulator 3 of this embodiment, in other words "a granulator 3 in which the rotating bed 4 rotates with the side wall 5 fixed," we analyzed how the "crushing strength immediately after granulation" and the "proportion of granules with a particle size of 4.75 mm or more to the total amount of granules (+4.75 mm ratio)" change when the granulation conditions "rotation speed of the rotating bed 4 (rotation speed of the device)" and "mixture amount of water in the raw material M (mixture set value)" are changed, and the results are shown in Table 2 and Figure 4.

Looking at the results in Table 2 and Figure 4, it can be seen that as the rotation speed of the granulation chamber 6 of the granulator 3 (the rotation speed of the rotating bed 4) is increased, the "proportion of granules with a particle size of 4.75 mm or more in the total amount of granules" gradually decreases. This is thought to be because if the rotation speed of the rotating bed 4 becomes too high, the granules collide with the side wall 5, damaging the granules that have been granulated with great care, and instead increasing the amount of granules with small particle sizes.

In other words, as shown in Figure 5, during granulation, in other words, the granulated material rolling on the rotating rotating bed 4 is not only subjected to the circumferential force associated with the rotation, but also, of course, the centrifugal force. Therefore, the resultant force of these forces acts on the granulated material as shown by the arrow in the figure. This resultant force acts in a direction that causes the granulated material to collide with the side wall 5, so the granulated material repeats the process of colliding with the side wall 5, being temporarily separated from the side wall 5 by the reaction force of the collision, and then colliding again as the material rotates.

As a result, as shown in Fig. 6, the granulated material rotates in a spiral shape, repeatedly colliding with and separating from the granulated material, and moves in circles around the rotation axis of the rotating bed 4. Therefore, when the rotation speed of the rotating bed 4 is high, the frequency of collisions with the side wall 5 increases, and it is considered that there is a high possibility that the granulated material will be damaged, resulting in a smaller particle size.
Also, looking at the results in Table 2 and Figure 4, when the "content of water in raw material M (set value)" is 10%, the "proportion of granules with a particle size of 4.75 mm or more in the total amount of granules" is extremely low at 19% or 29%. In contrast, when the "content of water in raw material M (set value)" is 11% or 12%, it is possible to increase the "proportion of granules with a particle size of 4.75 mm or more in the total amount of granules" to a target value of 95% or more depending on the rotation speed of the rotating bed 4.

On the other hand, regarding the rotation speed of the rotating bed 4, in experimental examples in which the "water content in the raw material M (set value)" is 11% or 12%, by setting the rotation speed to 50 rpm to 150 rpm, preferably 60 rpm to 110 rpm, it is possible to make the "proportion of granulated material with a particle size of 4.75 mm or more in the total amount of granulated material" equal to or more than the target value of 95%.
Furthermore, the granulated materials of Examples 1 to 11 in Table 2 were sieved through sieves with mesh sizes of 1.0 mm, 1.75 mm, 4.75 mm, 6.75 mm, 9.5 mm, and 19 mm, in the order of largest mesh size, and the results are shown in FIG. 5, which shows the extent of the granulated materials trapped on the sieves.

5, in Examples 10 and 11, in which the "content of water in raw material M (set value)" is 10%, the water is collected on sieves with openings of 1.0 mm, 1.75 mm, 4.75 mm, and 6.75 mm, but is hardly collected on sieves with openings of 9.5 mm and 19 mm. This shows that when the "content of water in raw material M (set value)" is less than 10%, the particle size of the granulated product becomes small, and it is not possible to granulate granulated products with a particle size of 4.75 mm or more.

However, in Examples 1 to 9, where the "amount of moisture in raw material M (set value)" is 11% or 12%, almost no granulated material is captured on sieves with openings of 1.0 mm or 1.75 mm, and most of the granulated material is captured on sieves with openings of 4.75 mm to 19 mm. From this, it is judged that if the "amount of moisture in raw material M (set value)" is set to 11% or 12%, the granulated material will grow to a sufficient particle size, and granulated material with a particle size of 4.75 mm or more can be produced with a high yield.

From the above, it is believed that if raw material M is a powder having a particle size of 20 μm or less, and water is mixed in an amount greater than 10 mass% and less than 12 mass% of the total amount, and the mixed raw material M is granulated in a granulator 3 having the above-mentioned configuration (a configuration having a granulation chamber 6 in which only the rotating bed portion 4 rotates) under granulation conditions of a rotation speed of 50 rpm to 150 rpm, preferably 60 rpm to 110 rpm, the result that "the proportion of particles with a particle size of 4.75 mm or more will be stably 95% or more" can be obtained.

The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. In particular, matters not expressly disclosed in the embodiments disclosed herein, such as operating conditions, operating conditions, various parameters, dimensions, weights, volumes of components, etc., do not deviate from the scope of what a person skilled in the art would normally do, and values that a person skilled in the art would easily imagine are used.

REFERENCE SIGNS LIST 1 Granulation equipment 2 Kneader 3 Granulator 4 Rotating bed 5 Side wall 6 Granulation chamber 7 Raw material feeder 8 Hopper 9 Shutter 10 Bottom wall 11 Through hole 12 Shaft 13 Driving motor 14 Power transmission mechanism 15 Discharge port 16 Granulation size adjustment plate M Raw material

Claims (3)

Powder dust having a particle size of 20 μm or less generated from an electric furnace for melting stainless steel or an electric furnace for oxidatively refining stainless steel is used as a raw material, and water and the raw material are charged into a kneading machine so that the water content is more than 10 mass% and 12 mass% or less of the total amount, and kneaded;
The mixed raw materials are fed into a granulator equipped with a granulation chamber having a rotating bed rotating at 60 to 90 revolutions per minute and a fixed side wall,
A granulation method characterized by: granulating the raw material while rotating the rotating bed in the granulation chamber for 2 minutes or more, thereby obtaining a granulated product in which particles having a particle size of 4.75 mm or more account for 95% or more of the total particles . The granulation method according to claim 1, characterized in that the powdery raw material contains FeO: 0.1 to 10 mass%, Fe ₂ O ₃ : 25 to 45 mass%, CaO: 2 to 15 mass%, SiO ₂ : 5 to 15 mass%, Al ₂ O ₃ : 0.1 to 15 mass%, MgO: 0.1 to 5 mass%, MnO: 2 to 15 mass%, Cr ₂ O ₃ : 10 to 20 mass%, with the remainder being iron and unavoidable impurities, and has a particle size of 20 μm or less. 3. The granulation method according to claim 2 , wherein all of the powdery raw materials used have a primary particle size of 10 μm or less.

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