KR20210024895A - Method for manufacturing pellet and binder, pallet mafufactured using the same - Google Patents

Abstract

According to an embodiment of the present invention, a manufacturing method of a pellet comprises: a process of recovering and preparing dust created during a ferronickel manufacturing process; a process of using first slag created during the ferronickel manufacturing process and byproducts created in processes other than the ferronickel manufacturing process to manufacture a binder; and a process of mixing assembling the dust and the binder to manufacture a pellet. Therefore, according to embodiments of the present invention, the manufacturing method of a pellet can manufacture a binder at low costs in comparison to a conventional technique. And the binder in accordance with embodiments of the present invention is used to manufacture a pellet which is a raw material for manufacturing ferronickel. A pellet with improved strength or strength similar to a pellet manufactured by using a conventional binder can be manufactured.

Description

Manufacturing method of pellets and binders, pellets manufactured using the same {METHOD FOR MANUFACTURING PELLET AND BINDER, PALLET MAFUFACTURED USING THE SAME}

The present invention relates to a method of manufacturing a pellet and a binder, and to a pellet manufactured using the same, and more particularly, to a method of manufacturing a pellet and a binder capable of reducing the cost for pellet manufacturing, and a pellet manufactured using the same. .

In general, molten steel for manufacturing stainless steel is manufactured through a refining process of dissolving solid ferronickel, ferrochrome, and scrap iron to produce a molten metal containing nickel and chromium, and removing carbon from the produced molten metal. .

Here, the process of preparing ferronickel is a process of drying nickel-containing ore (hereinafter, nickel ore), a process of sintering and pre-reduction of the dried nickel ore, and a process of charging and melting the nickel ore after firing and pre-reduction into an electric furnace. And a process of reducing, a process of removing sulfur (S) from the molten material, and a process of preparing the molten material having been degassed in a solid form.

On the other hand, in the process of drying the nickel ore, the process of firing and pre-reduction, and the process of melting and reducing, gas is generated, and the generated gas is discharged to the outside of each device.

Here, the gas discharged to the outside (hereinafter, exhaust gas) contains dust in the form of solid particles. In addition, this dust contains nickel oxide (NiO) in a high content.

Accordingly, dust is recycled as a source material for ferronickel production. At this time, in recycling the recovered dust, the dust is manufactured into pellets having a predetermined particle size or larger, and the pellets manufactured in the process of sintering and pre-reduction of nickel ore are recycled together.

On the other hand, when the strength of the pellets is weak, the pellets may be transported, or may be re-differentiated by being worn or crushed during firing and pre-reduction. For this reason, a large amount of dust may be generated in the process of firing and preliminary reduction.

Thus, in molding the dust into pellets, a binder for improving strength was added together to prepare it. However, there is a problem in that expensive inorganic binders are used as the binder, and the cost for manufacturing the pellets is high.

Korean Patent Registration KR0942109

The present invention relates to a method of manufacturing a pellet and a binder, which can reduce the cost for manufacturing a pellet, and a pellet manufactured using the same.

The present invention relates to a method of manufacturing a pellet and a binder capable of recycling the slag generated during the ferronickel manufacturing process, and to a pellet manufactured using the same.

A method of manufacturing a pellet according to an embodiment of the present invention comprises the process of recovering and preparing dust generated during the ferronickel manufacturing process; A process of manufacturing a binder using first slag, which is a slag generated during the ferronickel manufacturing process, and a by-product generated in a process other than the ferronickel manufacturing process, and a process of preparing a pellet by mixing and assembling the dust and the binder. .

The by-products include second slag and fly ash, which are slag generated during the production of molten steel for carbon steel production.

The process of recovering the dust may include a drying process of drying nickel ore during the ferronickel manufacturing process; A firing and preliminary reduction process of heating the nickel ore to calcinate and pre-reduce the nickel ore; A melting and reduction process of heating the nickel ore to generate a melt, and reducing the nickel ore; Dust generated during at least one of the processes is recovered.

The first slag contains 3.0% to 5.0% by weight of nickel oxide (NiO).

The first slag is prepared by recovering slag generated during a degassing process for removing sulfur (S) from the melt.

The second slag is prepared by recovering the slag generated in the refining process so that carbon (C) in molten steel is 1% by weight or less.

Of the entire binder, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight , Iron oxide (FeO) includes a process of controlling the mixing amount of the first slag, the second slag, and fly ash so that 10% by weight to 20% by weight is included.

In preparing the binder, a Na-based flux may be further mixed.

The Na-based flux may include Na ₂ CO ₃ .

Crushing each of the first slag, the second slag and the fly ash before mixing the first slag, the second slag, and the fly ash; And crushing the first mixture obtained by mixing the first slag, the second slag, and the fly ash. It includes at least one of.

The process of producing the pellet includes mixing the dust, the binder, and water, and the dust is 65% to 85% by weight based on the total weight% of the second mixture in which the dust, the binder, and water are mixed. , 5% to 15% by weight of the binder, and 10% to 20% by weight of water are mixed so as to be contained.

An embodiment of the present invention is a method of manufacturing a binder for manufacturing pellets, comprising: preparing a first slag, which is a slag generated during a ferronickel manufacturing process; A process of preparing a second slag, which is a slag generated during the manufacture of molten steel for carbon steel manufacturing; The process of preparing fly ash, an industrial by-product; And a process of preparing a binder by mixing the first slag, the second slag, and the fly ash.

The first slag is a slag containing 3.0% to 5.0% by weight of nickel oxide (NiO), and the second slag recovers slag generated in the refining process such that carbon (C) in molten steel is 1% by weight or less. To prepare.

The first slag is prepared by recovering slag generated during the degassing process to remove sulfur (S) from the melt, and the first slag is prepared by recovering slag having a particle diameter of 1 mm or less among the slag generated during the degassing process. do.

Of the entire binder, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight , Iron oxide (FeO) includes a process of controlling the mixing amount of the first slag, the second slag, and fly ash so that 10% by weight to 20% by weight of the iron oxide (FeO) is included, and the first slag and the second slag And the process of controlling the mixing amount of fly ash, with respect to the total weight% of the first mixture in which the first slag, the second slag, and the fly ash are mixed, the first slag is 10% to 30% by weight. , The second slag is mixed so that 10% to 30% by weight and 40% to 60% by weight of fly ash are contained.

The process of preparing the binder further includes a process of mixing Na-based flux, and based on the total weight% of the first mixture in which the first slag, second slag, fly ash, and Na-based flux are mixed, the first slag Is mixed to contain 10% to 30% by weight of the second slag, 10% to 30% by weight of the second slag, 40% to 60% by weight of fly ash, and 3% to 15% by weight of Na-based flux.

The Na-based flux may include Na ₂ CO ₃ .

The pellet according to an embodiment of the present invention includes a binder containing first slag, which is dust generated during the ferronickel manufacturing process and slag generated during the ferronickel manufacturing process.

The binder may include a second slag that is a slag generated during manufacturing molten steel for manufacturing carbon steel; And fly ash, which is an industrial by-product.

The first slag contains 3.0% to 5.0% by weight of nickel oxide (NiO).

The binder, silicon oxide (SiO ₂ ) of the entire binder is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 10% by weight 20% by weight and 10% to 20% by weight of iron oxide (FeO) are included.

The particle diameter of the pellet may be 15mm to 20.

The survival rate of the pellet may be 94% to 99.5%, the drop survival rate may be 92% to 95%.

The compressive strength of the pellets may be 1.8kgf to 2.1kgf.

In embodiments of the present invention, a binder is manufactured by recycling slag generated during the degassing process during the ferronickel manufacturing process, slag generated during the manufacturing process of molten steel for carbon steel manufacturing, and fly ash, which is an industrial by-product. Thus, it is possible to manufacture a binder at a lower cost than the conventional one. In addition, pellets, which are raw materials for ferronickel production, are manufactured using the binder according to the embodiments, and pellets having similar strength or improved strength to pellets manufactured using conventional binders may be manufactured.

Therefore, according to the embodiments of the present invention, while reducing the cost for manufacturing the pellet, it is possible to secure the strength of the pellet.

In addition, as the binder is manufactured by recycling the particulate degassing slag generated in the degassing process during the ferronickel manufacturing process, it is possible to save the inventory management cost of the particulate degassing slag that has not been recycled in the prior art.

In addition, since the particulate degassing slag contains nickel (Ni) and recycles it as a raw material for a binder to produce pellets, the binder acts as a source raw material for manufacturing ferronickel or for providing nickel (Ni). .

Accordingly, it is possible to reduce the amount of use of expensive nickel ore, which is a main raw material for manufacturing ferronickel and is a source material for providing nickel (Ni), thereby reducing the cost required for manufacturing ferronickel.

1 is a flowchart sequentially showing a method of manufacturing a pellet according to a first embodiment of the present invention.
2 is a diagram sequentially showing a general ferronickel manufacturing method.
3 is a graph showing the content of nickel oxide (NiO) according to the particle size of the slag generated by the degassing process during the ferronickel manufacturing process.
4 is a flowchart sequentially showing a method of manufacturing a pellet according to a second embodiment of the present invention.
5 is a table showing the survival rate (%), drop survival rate (%), compressive strength (kgf), and shape of the pellets prepared by the method according to Comparative Examples 1 and 2, and Examples 1 and 2. .

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a variety of different forms, only the present embodiments make the disclosure of the present invention complete, and the scope of the invention to those of ordinary skill in the art It is provided to inform you. In order to describe the embodiments of the present invention, the drawings may be exaggerated, and the same reference numerals in the drawings refer to the same elements.

The present invention relates to a method of manufacturing a pellet and a binder that can reduce the cost for manufacturing the pellet and recycle the slag generated during the ferronickel manufacturing process. More specifically, the present invention provides a method of manufacturing a pellet and a binder capable of reducing manufacturing cost while securing strength in manufacturing a pellet used as a raw material in manufacturing ferronickel.

1 is a flowchart sequentially showing a method of manufacturing a pellet according to a first embodiment of the present invention. 2 is a diagram sequentially showing a general ferronickel manufacturing method. 3 is a graph showing the content of nickel oxide (NiO) according to the particle size of the slag generated by the degassing process during the ferronickel manufacturing process.

Referring to FIG. 1, a method of manufacturing a pellet (S100) according to a first embodiment of the present invention includes a process of preparing dust generated during a ferronickel manufacturing process (S110), a process of manufacturing a binder (S120), and dust. And a process of preparing a pellet by mixing and assembling a binder (S130).

Here, the binder is a binder manufactured by the method according to the first embodiment.

The process of manufacturing the binder according to the first embodiment is a process of preparing slag (first slag) generated during the ferronickel manufacturing process, a process of preparing by-products generated in processes other than the ferronickel manufacturing process, the first slag and the binder The process of mixing, includes a process of preparing a binder using a mixture in which the first slag and by-products are mixed.

Here, the by-products mixed with the first slag may include slag (second slag) generated during the manufacturing process of molten steel for manufacturing carbon steel and fly ash, which is an industrial by-product.

Hereinafter, in describing the method of manufacturing the binder according to the first embodiment with reference to FIG. 1, the by-products mixed with the first slag are the second slag and fly ash generated during the manufacturing process of molten steel for carbon steel manufacturing. This will be explained with an example.

Referring to Figure 1, the process of manufacturing the binder according to the first embodiment (S120) is the process of preparing the first slag, which is the slag generated during the ferronickel manufacturing process (S121), generated during the manufacturing process of molten steel for carbon steel manufacturing. The process of preparing a second slag, which is a slag (S122), a process of preparing a fly ash, which is an industrial by-product (S123), a process of mixing the first and second slag, and the fly ash (S124), the first and And a process (S125) of manufacturing a binder using a mixture in which the second slag and fly ash are mixed.

In the present invention, in manufacturing the pellets, dust generated during the ferronickel manufacturing process is recycled as described above.

Hereinafter, prior to a detailed description of a method for manufacturing a pellet according to the first embodiment, a method for manufacturing ferronickel will be first described.

Referring to FIG. 2, the ferronickel manufacturing method includes a process of drying nickel ore (S10), a process of sintering and reducing the dried nickel ore (S20), and melting and reducing the calcined and reduced nickel ore. Hereinafter, a process of manufacturing a melt) (S30) and a degassing process (S40) of removing sulfur (S) from the melt are included.

Nickel ore is a source material for producing ferronickel or providing nickel (Ni) and iron (Fe), nickel oxide (NiO), calcium oxide (CaO), silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), an ore containing iron oxide (FeO), and may be an ore containing 1.5% to 2% by weight of nickel oxide (NiO). In addition, the nickel ore further includes manganese oxide (MnO), phosphorus pentoxide (P ₂ O ₅ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), carbon (C), and sulfur (S) in addition to the above-described components. I can. The nickel ore may be, for example, saprolite, and more specifically, the nickel ore may have a component composition as shown in the example of Table 1 below.

ingredient NiO CaO SiO ₂ Al ₂ O3 FeO MnO P ₂ O5 MgO TiO ₂ C S Content (% by weight) 1.8 0.24 38.92 0.97 10.77 0.62 0.02 33.16 0.02 0.054 0.0081

In the process of drying the nickel ore (S10), moisture contained in the nickel ore is removed. In this case, the drying apparatus for drying the nickel ore may be, for example, a rotary dryer, which is a means including a drum capable of being loaded and rotatable.

When a method of drying using a rotary dryer is described, nickel ore is charged into the drum, and the inside is heated to a temperature of, for example, 200°C while rotating the drum. Accordingly, moisture is removed from the nickel ore charged in the drum.

In heating the drum of the rotary dryer, for example, a method of supplying hot air into the drum may be used. Of course, the method of heating the inside of the drum is not limited to the above-described example, and may be applied to various methods. Further, the drying apparatus for drying the nickel ore is not limited to the rotary dryer described above, and various means capable of removing moisture from the nickel ore can be applied.

In the firing and reduction process (S20), the nickel ore is heat treated to increase the strength, and metal oxides such as nickel oxide (NiO) and iron oxide (FeO) contained in the nickel ore are converted to nickel (Ni) and iron (Fe). ).

The apparatus for sintering and reducing nickel ore (hereinafter, calcination apparatus) may be, for example, a rotary kiln, which is a means including a rotatable drum and a burner for generating a flame in the drum.

When a method of sintering and reducing nickel ore using a rotary kiln is described, a reducing agent including nickel ore and carbon (C), such as coal, is charged into the drum, and the inside is heated while rotating the drum. At this time, fuel gas is supplied to the burner, and a flame is generated into the drum through the burner, and the inside of the drum is heated to a temperature of 600°C to 800°C.

In this way, when the inside of the drum is heated to a temperature of 600° C. to 800° C., the nickel ore is fired to increase its strength, and metal oxides such as nickel oxide (NiO) and iron oxide (FeO) contained in the nickel ore are reduced.

In the above, in heating the inside of the drum of the rotary kiln, a method of generating a flame through a burner has been described. However, the present invention is not limited thereto, and various heating methods capable of heating the inside of the drum to a temperature of 600° C. to 800° C. may be applied. In addition, the firing apparatus for firing and reducing nickel ore is not limited to the above-described rotary kiln, and various means capable of firing and reducing nickel ore may be applied.

On the other hand, if the nickel ore is heated in this process (S20), it becomes easy to heat it to the target temperature in the melting and reduction process (S30) of the nickel ore, which is a subsequent operation. In addition, in the firing and reduction process (S20), the metal oxide is reduced by heating the nickel ore to a temperature of 600° C. to 800° C. as described above, which is the temperature in the melting and reduction process (S30) (1200° C. It is a lower temperature than the above). Accordingly, in the firing and reduction process (S20), the amount of reduced metal oxide is smaller than that in the melting and reduction process (S30).

Therefore, the reduction occurring in the firing and reduction process (S20) may be described as a reduction that is performed earlier or preliminarily compared to the reduction in the melting and reduction process (S30). Thus, the firing and reduction process (S20) carried out before the melting and reduction (S30) process is commonly referred to as a firing and preliminary reduction process (S20). More specifically, the firing and reduction process (S20) performed in the rotary kiln may be referred to as a firing and preliminary reduction process (S20).

The melting and reduction process (S40) melts the nickel ore and reduces metal oxides such as nickel oxide (NiO) and iron oxide (FeO) contained in the nickel ore. At this time, an electric furnace may be used to melt and reduce the nickel ore. Here, the electric furnace may have a configuration including a body having an internal space and an electrode for generating an arc as a heat source into the body.

When a method of melting and reducing nickel ore in an electric furnace is described, first, a reducing agent including nickel ore and carbon (C) is introduced into the main body of the electric furnace. Here, the reducing agent may be, for example, coal. When nickel ore and a reducing agent are charged into the body, an arc is generated into the body through an electrode. At this time, the nickel ore and the reducing agent are melted by arc heat, and the nickel ore is reduced to produce a melt, that is, a ferronickel melt.

On the other hand, the melt produced in the electric furnace contains a level of 4,000 ppm sulfur (S), and sulfur (S) is a factor that deteriorates the quality of products such as stainless steel. Therefore, when a melt is manufactured in an electric furnace, degassing is performed to remove sulfur (S) from the melt (S40), and at this time, it is necessary to adjust the sulfur (S) to 600 ppm or less.

Degassing to remove sulfur (S) from the melt can be carried out, for example, using a KR (Kanvara Reactor) device. Here, the KR apparatus may be a means including a rotatable impeller immersed in a melt charged into a ladle.

The evacuation operation in the KR apparatus can proceed as follows. When the melt produced in the electric furnace is charged into the ladle, the impeller is immersed with the melt in the ladle. Then, while rotating the impeller immersed in the melt, a degassing agent such as calcium oxide (CaO) is added to the melt.

When a degassing agent is introduced into the melt, CaS is generated by a reaction between sulfur (S) in the melt and the degassing agent, and the generated CaS flows into the slag floating on the surface of the melt. Accordingly, sulfur (S) is removed from the melt, and a ferronickel melt having a content of sulfur (S) of 600 ppm or less is prepared.

Meanwhile, during the manufacturing process of ferronickel as described above, gas is generated in each of the drying (S10), firing and pre-reduction (S20), and melting and reduction (S30) processes, and the gas is discharged to the outside of the device. That is, a drying device for drying nickel ore, a firing device for sintering and pre-reduction of nickel ore, and an electric furnace for melting and reducing each generate gas, and the generated gas is discharged to the outside of each device.

Here, the gas discharged to the outside (hereinafter, exhaust gas) contains dust in the form of solid particles. In addition, this dust contains nickel oxide (NiO) in an amount similar to that of nickel ore.

Therefore, dust generated during ferronickel production is recycled as a source material for producing ferronickel or providing nickel (Ni). That is, dust is recovered from exhaust gas discharged from a drying device, a firing device, and an electric furnace, and the recovered dust is recycled together with nickel ore as a source material.

To this end, a drying device, a firing device, and an exhaust gas treatment device for collecting dust from the exhaust gas are installed in each of the electric furnace, and the dust is recovered.

The exhaust gas treatment device is, for example, a pipe through which gas passes, a drive unit that generates a suction force so that gas is sucked into the pipe, a filter that passes gas and does not pass dust, and a filter that does not pass through the pipe. It may be a means including wealth.

And, in recycling the recovered dust, the nickel ore is recycled by putting it in the process (S20) of calcining and preliminary reduction. More specifically, nickel ore and recovered dust are put into a sintering apparatus, such as a rotary kiln, to be sintered and pre-reduced.

On the other hand, the dust recovered from the exhaust gas has a particle diameter of less than 0.1 mm, and when it is recycled as it is, there may be a problem that the amount of dust generated continuously increases as the number of processes for manufacturing ferronickel increases. More specifically, the amount of dust generated from the firing apparatus can be continuously increased.

Accordingly, the dust generated during ferronickel production is manufactured into pellets having a predetermined particle size or larger, and the manufactured pellets are introduced into a sintering device to be recycled as a source material.

However, even if the dust is made into pellets and recycled, if the strength of the pellets is weak, it may be worn and crushed to generate a large amount of dust again. That is, when the strength of the pellet is weak, it may be crushed by impact or abrasion applied to the pellet in the process of being transported for recycling to generate dust. In addition, nickel ore and coal are charged into the drum of the rotary kiln, which is a firing device, and fired and reduced while the drum rotates. At this time, when the strength of the pellets is weak, the pellets may be worn and crushed due to rotation of the drum and friction with nickel ore and coal, and dust may be generated.

As described above, when the strength of the pellet is weak, it is re-differentiated in the process of recycling it, so it is necessary to increase the strength of the pellet. To this end, in the manufacture of pellets by assembling dust, it is manufactured by adding a binder together. However, conventionally, an expensive inorganic binder is used as a binder, and there is a problem that the manufacturing cost of pellets is increased.

Accordingly, in the embodiment of the present invention, while securing the strength of the pellets, a binder that is inexpensive compared to the conventional binder is prepared, and pellets are manufactured using the binder.

The binder according to the first embodiment includes ferronickel degassing slag (first slag) generated during the degassing (S40) process during the ferronickel manufacturing process, slag (second slag) generated during the manufacturing process of molten steel for carbon steel production, and fly ash ( Fly ash).

_{Then, a binder containing silicon oxide (SiO 2} ), calcium oxide (CaO), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO) is prepared using the first slag, the second slag, and fly ash. At this time, with respect to the entire binder, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, and aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight %, iron oxide (FeO) is prepared to contain 10% by weight to 20% by weight. In addition, the binder may _{inevitably contain other components or impurities other than silicon oxide (SiO 2} ), calcium oxide (CaO), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO).

On the other hand, when at least one of silicon oxide (SiO ₂ ), calcium oxide (CaO), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO) among the binders is outside the respective content ranges suggested above, a conventional binder is used. It may not be possible to achieve a level of strength similar to that of the manufactured pellets.

That is, silicon oxide (SiO ₂ ) content in the binder is less than 30% by weight or more than 50% by weight, calcium oxide (CaO) is less than 15% by weight or more than 25% by weight, or aluminum oxide (Al ₂ O ₃ ) When the content is less than 10% by weight or more than 20% by weight, or the iron oxide (FeO) content is less than 10% by weight or more than 20% by weight, the strength of the pellets prepared using this binder is determined by using a conventional binder. It may not be possible to achieve a level of strength similar to that of the manufactured pellets.

As described above, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight, The preparation of a binder having an iron oxide (FeO) of 10% by weight to 20% by weight can be achieved by adjusting the mixing amount of each of the first slag, the second slag, and the fly ash.

Hereinafter, a method of manufacturing a binder according to a first embodiment of the present invention will be described with reference to FIG. 1.

Referring to Figure 1, the process of manufacturing a binder (S120) is a process of preparing the first slag, which is the slag generated in the degassing process (S40) during the ferronickel manufacturing process (S121), occurring during the manufacturing process of molten steel for carbon steel manufacturing The process of preparing the second slag, which is the slag (S122), the process of preparing the fly ash (S123), the process of mixing the first and second slag, and the fly ash (S124), the first and second It includes a process of manufacturing a binder using a mixture of slag and fly ash (S125).

In addition, before mixing the first and second slag and fly ash, the first and second slag and fly ash are crushed, respectively, or after mixing the first and second slag and fly ash, the mixture is crushed. It may further include a process.

The first slag is recovered and prepared from the slag generated in the degassing process (S40) during the ferronickel manufacturing process.

As described above, the degassing process (S40) is a process of removing sulfur (S) from the melt produced in an electric furnace, and a degassing agent such as calcium oxide (CaO) is introduced into the melt for degassing. When a degassing agent is introduced into the melt, CaS is generated by a reaction between sulfur (S) in the melt and the degassing agent, and the generated CaS flows into the slag floating on the surface of the melt. Thereafter, when the degassing is completed, the slag floating on the surface of the melt is removed to the outside.

The slag generated in the degassing process (S40) contains nickel oxide (NiO), calcium oxide (CaO), silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO). In addition, manganese oxide (MnO), phosphorus pentoxide (P ₂ O ₅ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), carbon (C), and sulfur (S) may be further included.

As described above, the slag generated in the degassing process (S40) contains nickel oxide (NiO), but since it contains nickel oxide (NiO) in a high content, it is recycled as a nickel source material for manufacturing ferronickel, Recycled by putting it into an electric furnace.

At this time, only slag with a particle diameter of 0.5 mm or more among the slag generated in the degassing process (S40) is recycled by putting it into an electric furnace, and degassing slag less than 0.5 mm is not recycled, but is managed as inventory. This is because, when fine-grained degassing slag of less than 0.5 mm is put into the electric furnace, the inputted large amount of slag is too light and is sucked into the piping of the exhaust gas treatment device as it is.

On the other hand, the content of nickel oxide (NiO) is different depending on the particle size of the slag. That is, as shown in Fig. 3, the slag with a particle diameter of 10 mm or more contains 10% by weight or more of nickel oxide, and the slag with a particle diameter of 5 mm or more and less than 10 mm contains 6.6% by weight of nickel oxide, and the slag with a particle diameter of 3 mm or more and less than 5 mm is oxidized. Nickel oxide is 2.9% by weight, for slag with a particle diameter of 1mm or more and less than 3mm, nickel oxide is 3.3% by weight, for slag with a particle diameter of 0.5mm or more and less than 1mm, nickel oxide is 1.9% by weight, and for slag with a particle diameter of less than 0.5mm, nickel oxide (NiO ) Is contained in an amount of 3.0% by weight to 5% by weight, more specifically 4.0% by weight.

Here, the content of nickel oxide among the slag with a particle diameter of less than 0.5 mm is less than that of slag with a particle diameter of 10 mm or more, and slag with a particle diameter of 5 mm or more and less than 10 mm, but the content is high about 3.0% to 5% by weight, so that the recycling value is sufficient. Do.

However, among the slag generated in the current degassing process (S40), degassing slag of less than 0.5 mm has not been recycled.

Accordingly, in the embodiment, the slag having a particle diameter of less than 0.5 mm among the slag generated in the degassing process (S40) is recycled as a raw material for manufacturing the binder. Accordingly, the first slag may be described as a slag having a particle diameter of 0.5 mm or less among the slag generated in the degassing process (S40) during the ferronickel manufacturing process.

And, in the embodiment, among the slag generated in the degassing process (S40), the slag having a particle diameter of less than 0.5 mm, that is, the first slag, in addition to nickel oxide (NiO), calcium oxide (CaO), silicon oxide (SiO ₂ ) , Iron oxide (FeO) is included. Here, calcium oxide (CaO) may be contained in an amount of 25% to 30% by weight, silicon oxide (SiO ₂ ) may be contained in an amount of 15% to 20% by weight, and iron oxide (FeO) may be contained in an amount of 15% to 20% by weight. In addition, the first slag includes aluminum oxide (Al ₂ O ₃ ), manganese oxide (MnO), phosphorus pentoxide (P ₂ O ₅ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), carbon (C), sulfur ( S) may be included, and a specific example of the first slag is shown in Table 2.

The first slag is a raw material for manufacturing a binder, and _{contains relatively large amounts of calcium oxide (CaO), silicon oxide (SiO 2} ), and iron oxide (FeO) compared to other components. Accordingly, the first slag may be described as a raw material for adjusting each of calcium oxide (CaO), silicon oxide (SiO _{2 ), and iron oxide (FeO) of the binder to a target content.}

In addition, as described above, since the first slag contains 3.0% by weight to 5% by weight of nickel oxide (NiO), the first slag is also used as a source material for producing ferronickel or for providing nickel (Ni). Works.

ingredient CaO content (% by weight) SiO ₂ content (% by weight) Al ₂ O ₃ content (% by weight) FeO content (% by weight) MnO content (% by weight) P ₂ O ₅ content (% by weight) MgO content (% by weight) TiO ₂ content (% by weight) C content (% by weight) S content (% by weight) Inorganic binder 12.80 40.63 12.01 13.16 0.32 0.19 3.43 1.36 1.49 0.089 First slag 28.87 17.23 2.78 17.83 0.49 0.05 6.64 0.05 3.68 11.3 Second slag 38.06 12.45 24.32 22.72 3.49 0.28 7.54 0.79 0.023 0.052 Fly ash 1.58 74.17 15.04 4.60 0.42 0.07 1.77 0.77 0.054 0.0062

The second slag is a slag generated during the manufacturing process of molten steel for carbon steel.

In general, the manufacturing process of molten steel for carbon steel is a preliminary refining process that removes sulfur (S) from the molten iron, a converter refining process that removes carbon (C) from the molten iron after preliminary refining, and the molten steel at which the converter refining has ended. It further removes carbon (C) from and includes a secondary refining process to remove (deoxidize) oxygen.

Preliminary refining may be performed, for example, using a KR (Kanvara Reactor) device, and in this case, a degassing agent such as calcium oxide (CaO) may be used.

The converter refining process is a process of charging molten iron into a converter and blowing oxygen into the molten iron through a lance to remove C (carbon) in the molten iron to 4% by weight or less.

The secondary refining can be carried out in a vacuum degassing unit RH (Rheinstahl Huttenwerke & Heraeus) unit. Here, the vacuum degassing device is provided on the upper side of the ladle, a vacuum bath having an internal space, a lance provided to penetrate the upper part of the vacuum bath, and a pair of immersion tubes mounted to be spaced apart in the width direction of the vacuum bath from the lower part of the vacuum bath. Includes. One of the pair of immersion tubes is inserted and installed so that a nozzle passes therethrough, and an inert gas such as Ar gas is injected into the inside thereof. Here, among the pair of immersion pipes, the immersion pipe into which a nozzle is inserted or supplied with an inert gas is a rising pipe, and the other is called a descending pipe.

The vacuum tank degassing apparatus reduces the pressure in the vacuum tank to several torr or less, raises the molten steel in the ladle into the vacuum tank, and then circulates the molten steel from the riser to the downcomer. When the molten steel in the ladle is circulated or refluxed to pass through the riser and the downcomer, oxygen is blown through the lance to adjust carbon (C) in the molten steel to 1% by weight or less.

When the secondary refining in the vacuum tank degassing device is completed, the slag floating above the molten steel in the ladle is removed.

The slag generated by secondary refining during the manufacturing process of molten steel for carbon steel contains calcium oxide (CaO), silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO). Here, calcium oxide (CaO) is 35% to 40% by weight, silicon oxide (SiO ₂ ) is 10% to 15% by weight, aluminum oxide (Al ₂ O ₃ ) is 20% to 25% by weight, iron oxide ( FeO) may be contained in an amount of 20% to 25% by weight. In addition, manganese oxide (MnO), phosphorus pentoxide (P ₂ O ₅ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), carbon (C), and sulfur (S) may be further included. A specific example of slag generated by secondary refining during the manufacturing process of molten steel for carbon steel is shown in Table 2.

In the first embodiment, the slag generated by secondary refining during the manufacturing process of molten steel for carbon steel is recycled as a raw material for manufacturing a binder. Accordingly, the second slag may be described as a slag generated by secondary refining during the manufacturing process of molten steel for carbon steel.

This second slag is a raw material for manufacturing a binder, _{and contains relatively large amounts of calcium oxide (CaO), silicon oxide (SiO 2} ), aluminum oxide (Al ₂ O ₃ ), and iron oxide (FeO) compared to other components. have. Accordingly, the second slag may be described as a raw material for controlling each of calcium oxide (CaO), silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O _{3 ), and iron oxide (FeO) of the binder to a target content.}

Fly ash is a raw material containing 60% to 80% by weight of silicon oxide (SiO ₂ ) and 10% to 20% by weight of _{aluminum oxide (Al 2} O _{3 ).} It may be an industrial by-product generated when burned.

Fly ash contains silicon oxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), calcium oxide (CaO), and iron oxide (FeO). In addition, the fly ash may further include manganese oxide (MnO), phosphorus pentoxide (P ₂ O ₅ ), magnesium oxide (MgO), titanium oxide (TiO ₂ ), carbon (C), and sulfur (S). And, a specific example of the fly ash is shown in Table 2.

Such fly ash is a raw material for preparing a binder, and contains a relatively large amount of _{silicon oxide (SiO 2} ) and aluminum oxide (Al ₂ O _{3) compared to other components.} Accordingly, fly ash may be described as a raw material for controlling each of _{silicon oxide (SiO 2} ) and aluminum oxide (Al ₂ O _{3) of a binder to a target content.}

When the first and second slag and fly ash as described above are prepared (S121, 122, 123), each of the first and second slag and fly ash is crushed before mixing them so that the particle diameter is 100 μm or less. It is desirable to do so. Of course, the present invention is not limited thereto, and after mixing the first and second slag and fly ash, the mixture may be crushed to have a particle diameter of 100 μm or less.

This is to improve the assembling property when preparing pellets by mixing the binder and dust and then assembling them.

When the first and second slag and fly ash as described above are prepared (S121, 122, 123), they are mixed (S124). At this time, with respect to the total weight% of the mixture of the first slag, the second slag, and fly ash (hereinafter, referred to as the first mixture), the first slag is 10% to 30% by weight, and the second slag is 10% to 30% by weight. Mix so that the weight percent, fly ash is 40 to 60% by weight.

On the other hand, in the case where at least one of the first slag, the second slag, and the fly ash of the first mixture is outside the respective content ranges set forth above, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO ) Is 15% by weight to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% by weight to 20% by weight, iron oxide (FeO) is 10% by weight to 20% by weight it may be difficult to prepare a binder.

That is, in the first mixture, the content of the first slag is less than 10% by weight or more than 30% by weight, the content of the second slag is less than 10% by weight or more than 30% by weight, or the content of fly ash is 40% by weight. If less than% or more than 60% by weight, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10 It may be difficult to prepare a binder having 10% to 20% by weight of iron oxide (FeO) by weight. In addition, when such a binder is used, it may not be possible to secure a level of strength similar to that of pellets manufactured using a conventional binder.

Accordingly, in the embodiment, the first slag is 10% to 30% by weight, the second slag is 10% to 30% by weight, and the fly ash is mixed so that it is 40% to 60% by weight.

When the first and second slag and fly ash are mixed (S124), the mixture (ie, the first mixture) and water are mixed to prepare a binder (S125).

In this way, when a binder is manufactured using the first and second slag and fly ash, the prepared dust, the prepared binder, and water are mixed to prepare a pellet (S130).

A granulator for producing pellets by assembling dust may be, for example, a pelletizer including a fan in which raw materials to be assembled are charged or accommodated, and a driving part for rotating the fan.

When a method of manufacturing pellets using a pelletizer is described, first, dust, a binder, and water are introduced into the fan, and the fan is rotated by operating a driving unit. At this time, dust, a binder, and water are mixed and assembled by the rotation of the fan, so that a granulated product having a predetermined size, that is, pellets is manufactured.

In preparing a pellet by mixing dust, a binder, and water, the dust is 65% to 85% by weight and the binder is 5% to 15% by weight of the total mixture of dust, binder, and water (hereinafter, referred to as the second mixture). %, water is mixed so that 10% to 20% by weight.

On the other hand, for example, in the case where at least one of the second mixture in which dust, binder, and water are mixed is out of the respective content ranges set forth above, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO ) Is 15% by weight to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% by weight to 20% by weight, iron oxide (FeO) is 10% by weight to 20% by weight it may be difficult to prepare a binder. In addition, when such a binder is used, it may not be possible to secure a level of strength similar to that of pellets manufactured using a conventional binder.

In addition, the particle size of the pellets is preferably adjusted to 15mm to 20mm, which can be adjusted by adjusting at least one of the assembly time, the rotational speed of the fan, and the loading amount of the raw material (second mixture) charged into the fan.

4 is a flowchart sequentially showing a method of manufacturing a pellet according to a second embodiment of the present invention.

In the above-described first embodiment, in preparing the binder, the first slag, the second slag, and the fly ash were mixed.

However, the present invention is not limited thereto, and Na-based flux may be further mixed as shown in FIG. 4 in preparing the binder. That is, the process of manufacturing the binder according to the second embodiment is the process of preparing the first slag, which is the slag generated during the degassing process during the ferronickel manufacturing process (S121), 2 Process of preparing slag (S122), process of preparing fly ash (S123), process of preparing Na-based flux (S126), first slag, second slag, fly ash, and Na-based flux A process of mixing (S126), a process of manufacturing a binder using a mixture of first slag, second slag, fly ash, and Na-based flux (S125).

Here, the Na-based flux is added to improve the strength of the pellet, and the Na-based flux may be, for example, a material containing _{Na 2} CO _3.

In addition, in the entire mixture (first mixture) in which the first slag, the second slag, fly ash, and Na-based flux are mixed, it is preferable to mix the Na-based flux so that the content is 3% to 15% by weight. . That is, in the entire first mixture, the first slag is 10% to 30% by weight, the second slag is 10% to 30% by weight, fly ash is 40% to 60% by weight, Na-based flux is 3% by weight And mix so as to be 15% by weight.

On the other hand, for example, when the content of the Na-based flux in the first mixture is less than 3% by weight, the effect of improving the strength of the pellet may be insignificant compared to the case of using the binder according to the first embodiment in which the Na-based flux is not mixed. Conversely, when the content of the Na-based flux exceeds 15% by weight, the rate of increase in the strength of the pellets relative to the input amount is small. Thus, from the viewpoint of the expression of the effect compared to the input, it is preferable that the content of the Na-based flux is 15% by weight or less.

5 is a table showing the survival rate (%), drop survival rate (%), compressive strength (kgf), and shape of the pellets prepared by the method according to Comparative Examples 1 and 2 and Examples 1 and 2 .

Here, the pellets according to Comparative Examples 1 and 2 and Examples 1 and 2 are pellets manufactured by recovering dust generated during the ferronickel manufacturing process and assembling the same. In addition, the first comparative example is a pellet without using a binder, and the second comparative example is a pellet prepared using a conventional inorganic binder.

Further, the pellet according to the first embodiment is a pellet produced by using the binder according to the first embodiment, that is, a binder prepared by mixing the first slag, the second slag, and the fly ash. At this time, in the entire first mixture in which the first slag, the second slag, and the fly ash are mixed, the first slag is 10% to 30% by weight, the second slag is 10% to 30% by weight, and the fly ash is 40 It was mixed so that it might be weight% to 60 weight %.

And, the pellet according to the second embodiment is a pellet produced by using the binder according to the second embodiment, that is, a binder prepared by mixing the first slag, the second slag, fly ash, and Na-based flux. At this time, in the total mixture of the first slag, the second slag, fly ash and Na-based flux, the first slag is 10% to 30% by weight, the second slag is 10% to 30% by weight, fly ash Was mixed so that 40% to 60% by weight and 3% to 15% by weight of Na-based flux.

In FIG. 5, the survival rate is when pellets are charged into the drum of a rotary kiln, which is a firing device, and the drum is heated at a temperature of 600° C. for 20 minutes while rotating the drum at a speed of 5 rpm to sinter and reduce the sintering and reduction. This is the weight ratio of pellets with a particle diameter of 10 mm or more. That is, the weight ratio of pellets having a particle diameter of 10 mm or more after completion of firing and reduction to the total weight of the pellets charged into the drum of the rotary kiln.

In addition, the drop survival rate is a weight ratio of a pellet having a weight of 10 mm or more when the pellet is dropped five times in succession from a height of 2.5 m. In other words, it is the weight ratio of the pellets having a particle diameter of 10 mm or more among the dropped pellets.

Compressive strength (kgf) means resistance to weight per unit, and compressive strength is a measure of the maximum load until the pellet is destroyed by applying a pressure at a constant speed to the top of the pellet placed on the roll press device.

Referring to FIG. 5, the survival rate, drop survival rate, and compressive strength of the pellets according to the second comparative example, the first and second examples are significantly higher than that of the first comparative example. In addition, when comparing the amount of fine-grained pellets having a particle diameter of 5 mm or less among the manufactured pellets, the second comparative example, the first and second examples are significantly smaller than that of the first comparative example.

In addition, when comparing the second comparative example with the first and second examples, the survival rate of the pellets according to the first and second examples is higher than that of the second comparative example manufactured using an expensive inorganic binder.

In addition, the drop survival rate and compressive strength of the pellets according to the first embodiment are small in a predetermined range compared to the second comparative example, but the drop survival rate and compressive strength are similar to those of the second comparative example, and a problem occurs during ferronickel production. It is a level that does not make it. In addition, it can be seen that the drop survival rate and compressive strength of the pellets according to the second embodiment are higher in a predetermined range than in the second comparative example using an expensive inorganic binder.

From this, it can be seen that the pellets manufactured using the binder manufactured by the method according to the embodiments of the present invention have similar or improved strength to the pellets manufactured using the conventional binder.

In addition, it can be seen that the binder according to the second embodiment can produce pellets having a higher survival rate, drop survival rate, and compressive strength than the binder according to the first embodiment.

As described above, in an embodiment of the present invention, a binder is manufactured by recycling slag generated during the degassing process during the ferronickel manufacturing process, slag generated during the manufacturing process of molten steel for carbon steel manufacturing, and fly ash, which is an industrial by-product. Thus, it is possible to manufacture a binder at a lower cost than the conventional one.

In addition, pellets, which are raw materials for ferronickel production, are manufactured using the binder according to the embodiments, and pellets having similar strength or improved strength to pellets manufactured using conventional binders may be manufactured.

Therefore, according to the embodiments of the present invention, it is possible to secure strength while reducing the cost for manufacturing pellets.

In addition, as the binder is manufactured by recycling the particulate degassing slag generated in the degassing process during the ferronickel manufacturing process, it is possible to save the inventory management cost of the particulate degassing slag that has not been recycled in the prior art.

In addition, since the particulate degassing slag contains nickel (Ni) and recycles it as a raw material for a binder to produce pellets, the particulate degassing slag is a source material for producing ferronickel or providing nickel (Ni). Acts as

Accordingly, it is possible to reduce the amount of use of expensive nickel ore, which is a main raw material for manufacturing ferronickel and is a source material for providing nickel (Ni), thereby reducing the cost required for manufacturing ferronickel.

S100: manufacturing process of pellets
S120: finder manufacturing process

Claims (24)

A process of recovering and preparing dust generated during the ferronickel manufacturing process;
Manufacturing a binder using first slag, which is a slag generated during the ferronickel manufacturing process, and a by-product generated in a process other than the ferronickel manufacturing process; And
Mixing and assembling the dust and the binder to produce a pellet;
A method for producing a pellet comprising a. The method according to claim 1,
The by-product is a method for producing a pellet comprising a second slag and fly ash, which is a slag generated during the production of molten steel for manufacturing carbon steel. The method according to claim 1,
The process of recovering the dust,
During the ferronickel manufacturing process, a drying process of drying the nickel ore; A firing and preliminary reduction process of heating the nickel ore to fire and pre-reduction; A melting and reduction process of heating the nickel ore to generate a melt, and reducing the nickel ore; A method for producing a pellet for recovering dust generated during at least one of the processes. The method according to claim 1,
The first slag is a slag containing 3.0% to 5.0% by weight of nickel oxide (NiO). The method of claim 4,
The first slag is a method for producing a pellet by recovering slag generated during a degassing process for removing sulfur (S) from the melt. The method according to claim 2,
The second slag is a method for producing a pellet obtained by recovering the slag generated in a refining process in which carbon (C) in molten steel is 1% by weight or less. The method according to claim 2,
Of the entire binder, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight , Iron oxide (FeO) 10% by weight to 20% by weight of the pellets manufacturing method comprising the process of controlling the mixing amount of the first slag, the second slag, and fly ash. The method of claim 3,
In preparing the binder, a method of producing a pellet further mixing Na-based flux. The method of claim 8,
The Na-based flux is a method for producing a pellet containing _{Na 2} CO _3. The method according to claim 2,
Crushing each of the first slag, the second slag and the fly ash before mixing the first slag, the second slag, and the fly ash; And
Crushing a first mixture of the first slag, the second slag, and fly ash;
A method for producing a pellet comprising at least one of. The method according to any one of claims 1 to 10,
The process of preparing the pellets,
Including the process of mixing the dust, the binder and water,
With respect to the total weight% of the second mixture in which the dust, binder and water are mixed, the dust contains 65% to 85% by weight, the binder is 5% to 15% by weight, and the water is 10% to 20% by weight. A method for producing a pellet to be mixed as possible. As a method for producing a binder for producing pellets,
Preparing a first slag, which is a slag generated during a ferronickel manufacturing process;
A process of preparing a second slag, which is a slag generated during the manufacture of molten steel for carbon steel manufacturing;
The process of preparing fly ash, an industrial by-product; And
Manufacturing a binder by mixing the first slag, the second slag, and fly ash;
Method for producing a binder comprising a. The method of claim 12,
The first slag is a slag containing 3.0% to 5.0% by weight of nickel oxide (NiO),
The second slag is a method of manufacturing a binder prepared by recovering slag generated in a refining process in which carbon (C) in molten steel is 1% by weight or less. The method of claim 13,
The first slag is prepared by recovering slag generated during a degassing process for removing sulfur (S) from the melt,
The first slag is a method of manufacturing a binder in which slag having a particle diameter of 1 mm or less is recovered from among the slag generated during the degassing process. The method of claim 12,
Of the entire binder, silicon oxide (SiO ₂ ) is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 20% by weight , Iron oxide (FeO) includes a process of controlling the mixing amount of the first slag, the second slag and fly ash so that 10% by weight to 20% by weight is included,
The process of controlling the mixing amount of the first slag, the second slag, and fly ash,
The first slag is 10% to 30% by weight, the second slag is 10% to 30% by weight, fly ash A method for producing a binder to be mixed to contain 40% by weight to 60% by weight. The method of claim 12,
The process of preparing the binder further includes a process of mixing Na-based flux,
With respect to the total weight% of the first mixture in which the first slag, second slag, fly ash and Na-based flux are mixed, the first slag is 10% to 30% by weight, and the second slag is 10% to 30% by weight %, fly ash is 40% by weight to 60% by weight, Na-based flux is 3% by weight to 15% by weight of the mixture so as to be contained in a method for producing a binder. The method of claim 16,
The Na-based flux is a method of manufacturing a binder containing _{Na 2} CO _3. Dust generated during the ferronickel manufacturing process; And
A binder containing the first slag, which is the slag generated during the ferronickel manufacturing process;
Pellets containing. The method of claim 18,
The binder,
Second slag, which is the slag generated during the manufacture of molten steel for carbon steel manufacturing; And
Fly ash, an industrial by-product;
Pellets containing. The method of claim 18,
The first slag is a pellet containing 3.0% to 5.0% by weight of nickel oxide (NiO). The method of claim 18,
The binder, silicon oxide (SiO ₂ ) of the entire binder is 30% to 50% by weight, calcium oxide (CaO) is 15% to 25% by weight, aluminum oxide (Al ₂ O ₃ ) is 10% to 10% by weight Pellets containing 20% by weight and 10% to 20% by weight of iron oxide (FeO). The method of claim 18,
Pellets having a particle diameter of 15 mm to 20. The method of claim 18,
Pellets with a survival rate of 94% to 99.5% and a drop survival rate of 92% to 95%. The method of claim 18,
Pellets with a compressive strength of 1.8kgf to 2.1kgf.

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