JP5711189B2 - High quality sorting method of layered clay minerals by wet grinding and classification - Google Patents

Description

  The present invention relates to a high-grade sorting method for layered clay minerals, and more specifically, impure minerals are classified by using wet grinding and classification techniques for wax stones and talc, which are typical layered clay minerals widely used as industrial raw material minerals. The present invention relates to a high-grade sorting method for layered clay minerals by wet pulverization and classification, which can recover high-grade wax or talc fine particles by removing.

  Waxite and talc are layered silicate-aluminum minerals and are used in various raw materials such as paints, plastics, cosmetics, and pharmaceuticals. However, like most ores produced in nature, contamination of impure minerals such as quartz causes the limitation of the range of use and the price reduction.

  Impure minerals such as quartz cause a reduction in the chemical composition of wax and talc minerals, and cause wear of crushing equipment such as crushers and crushing media in the fine crushing process for use in industrial fillers. It becomes. Such abrasion of the crusher shortens the life of the apparatus, and the worn components may cause contamination of the pulverized product, resulting in a reduction in product quality. The pulverization process for processing into an industrial filler is known as a unit process that consumes a large amount of energy.

In a unit operation that consumes a large amount of energy, hard impurities such as quartz cause a significant increase in input energy, which adversely affects product quality and energy consumption. Therefore, most mine relies on manual selection methods that determine the size of coarse particles of a certain size or larger by hand with the naked eye in order to solve the problems caused by contamination.

  However, the manual selection method is a method of selecting by hand with the naked eye, and can be applied only to particles of a certain size or larger, that is, approximately 20 mm or more. Is possible. Therefore, such a manual selection method is known as a primitive method having characteristics of low sorting efficiency and small processing amount.

  On the other hand, specific gravity sorting is a method in which ore is pulverized to a size of 20 to 100 mesh (mesh) or less and poured into water or precipitated, and separated using the difference in specific gravity between the ore mineral and impure mineral. It is limited to high iron-containing minerals.

  In the floating sorting method, a raw ore is crushed in water to a certain size or less, approximately 35 to 200 mesh, and then a collector is used to make the surface of the particles to be removed hydrophobic. After adding a frother, which is a reagent that can generate bubbles while stirring with a stirrer so that the reagent and the ore can be fully contacted, air is injected into the water from the outside into the bubbles This is a sorting method that separates ore minerals and impure minerals by discharging attached ore particles to the outside.

  Such a floating sorting method is known as a technique with excellent sorting efficiency. However, in the case of the floating sorting method, a water trapping agent, a foaming agent, and various regulators must be used in the sorting process, and the sorting device is complicated and the facility cost is expensive. With difficulty. In particular, since there is a risk of generating wastewater due to the water capturing agent and foaming agent used in each process, many environmental restrictions are followed.

  The related prior literature includes Korean Published Patent No. 10-2008-0105314 (published 2008.12.04), which describes a method for removing iron from wax.

  The purpose of the present invention is to improve the sorting efficiency, such as the quality improvement rate and the actual yield, there is no wastewater to be discharged, the process is simple, the economic burden is small, and the manual selection method and the floating selection method. It is an object of the present invention to provide a high-grade sorting method for layered clay minerals by wet pulverization and classification, which can solve the above problems simultaneously.

  Also, the object of the present invention is to apply impure minerals such as quartz mixed in layered silicate-aluminum minerals such as wollastonite and talc, which are used in industrial fillers, and assembling pyrite to wet grinding and classification processes. It is possible to produce high-purity concentrates by improving the quality improvement rate and yield and reducing input energy, and can be used for high-purity raw materials such as cosmetics and pharmaceutical raw materials as well as existing paints and plastics. It is an object of the present invention to provide a high-quality sorting method for layered clay minerals by wet grinding and classification capable of producing industrial fillers.

In order to achieve the above object, a high-grade screening method for a layered clay mineral according to the first embodiment of the present invention is to convert a granite or talc ore containing impure minerals including quartz or pyrite into particles having a diameter of 30 mm or less. A step of primary crushing, a step of secondary crushing the primary crushed particles, a step of primary crushing the secondary crushed particles using a Raymond mill, and the primary crushing The method includes a step of first classifying the particles into coarse particles and fine particles by wet classification using a wet classifier.

In order to achieve the above object, a high-grade screening method for a layered clay mineral according to a second embodiment of the present invention uses a jaw crusher to remove wax or talc ore containing impure minerals including quartz or pyrite. A step of primary crushing into particles having a diameter of 30 mm or less, a step of secondary crushing the primary crushed particles using a hammer crusher, and a step of crushing the secondary crushed particles into a Raymond mill. ) the step of primary pulverization using a step of primary classification in the wet classification to coarse particles and fine particles by a wet classifier and the primary ground particles, of the primary classification particles, secondary steps, and the second ground particles in the wet classification to coarse particles by a wet classifier and particulates secondary pulverization with the attrition mill (attrition mill) microparticles Characterized in that it comprises a step of grade.

  The high-grade screening method for layered clay minerals by wet pulverization and classification according to the present invention has an advantage that water can be reused after the product is recovered, since a reagent is not used in the treatment process as well as the quality improvement rate.

  In particular, the high-grade sorting method for layered clay minerals by wet grinding and classification according to the present invention performs fine grain grinding for sorting, so grinding and particle size separation for filler production are completed, and separate grinding and classification. Since it has the advantage of not requiring a process, it can be said to be an advantageous sorting method in terms of economy, such as energy input and facility costs.

It is a schematic diagram which shows the high quality selection method of the layered clay mineral according to 1st Embodiment of this invention. It is a schematic diagram which shows more specifically the high quality sorting method of the layered clay mineral according to the first embodiment of the present invention. It is a schematic diagram which shows the high quality selection method of the layered clay mineral according to 2nd Embodiment of this invention. It is a schematic diagram which shows more specifically the high quality sorting method of layered clay mineral according to the second embodiment of the present invention.

  Advantages and features of the present invention and methods of achieving them will be apparent with reference to the embodiments described in detail below in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various forms different from each other. The embodiments merely make the disclosure of the present invention complete, and the technology to which the present invention belongs. It is provided to provide full knowledge of the scope of the invention to those skilled in the art and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

  Hereinafter, a high-grade sorting method for layered clay minerals by wet grinding and classification according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic diagram showing a high-grade sorting method for layered clay minerals according to the first embodiment of the present invention, and FIG. 2 more specifically shows a high-grade sorting method for layered clay minerals according to the first embodiment of the present invention. It is a schematic diagram shown.

  Referring to FIG. 1, the high-grade screening method for layered clay mineral according to the illustrated first embodiment includes a primary crushing step (S110), a secondary crushing step (S120), a primary crushing step (S130), and 1 A next classification step (S140) is included.

<Primary crushing>
In the primary crushing step (S110), the cinder or talc ore containing impure minerals including quartz or pyrite is primarily crushed. In this case, the wollastonite or talc ore may contain a small amount of iron oxide, sphalerite, titanium iron stone, etc. that act as a cause of whiteness in the processing process in addition to quartz and pyrite.

  At this time, it is appropriate that the primary crushing is performed by crushing particles having a diameter of 30 mm or less using a jaw crusher.

<Secondary crushing>
In the secondary crushing step (S120), the primary crushed particles are secondarily crushed using a hammer crusher. At this time, primary crushing and secondary crushing are pretreatment steps for crushing, which will be described later, and the particle size of the particles produced in each crushing step can be changed according to crushing and classification efficiency.

<Primary grinding>
In the primary pulverization step (S130), the secondary crushed particles are primarily pulverized. At this time, the primary pulverization is preferably performed using a Raymond mill in which impact force and friction force mainly act on the secondary crushed particles. In particular, although the pulverized particle size may vary depending on the thickness of the impure mineral, it is appropriate to pulverize into particles having a diameter of 200 mesh or less.

<Primary classification>
In the primary classification step (S140), the primary pulverized particles are wet-classified by a wet classifier to be primary classified into coarse particles and fine particles .

In such a primary classification step (S140), coarse particles and fine particles are determined to be coarse particles based on a diameter of 325 mesh, and particles having a diameter of 325 mesh or less are determined to be fine particles. Sizing. At this time, coarse particles contain a larger amount of impure minerals than fine particles .

On the other hand, high-quality screening method of the layered clay mineral according to the first embodiment of the present invention, reground step (S150) and the re-classification steps (S160) may further include.

< Reground >
Referring to FIG. 2, in the re-pulverization step (S150), coarse particles among the primary classified particles are re-ground using an attrition mill. In particular, in this step, the concentration of the mineral liquid and the operating conditions of the crusher may differ depending on the conditions of the raw material minerals to be charged. It is difficult to define uniformly, but the attrition mill (attrition mill) standard conditions are preferably applied. That is, it is preferable to maintain the ball charging amount of 20 to 40 v / v% of the mill internal volume, the ball size of 3 to 6 mm, and the mineral liquid concentration of 40 to 60 wt%.

< Reclassification >
Re classified again classifying step (S160) in re-ground particles in a wet classification to coarse particles by a wet classifier and particulate. At this time, coarse particles classified by the re-classifying step (S160) is suitable for use in ceramic material.

  The above-described high-grade screening method for layered clay mineral according to the first embodiment of the present invention has the advantage that the water can be reused after the product is recovered, since the reagent is not used in the process as well as the quality improvement rate. There is.

  FIG. 3 is a schematic diagram illustrating a high-grade sorting method for layered clay mineral according to the second embodiment of the present invention, and FIG. 4 illustrates the high-grade sorting method for layered clay mineral according to the second embodiment of the present invention more specifically. It is a schematic diagram shown.

  Referring to FIG. 3, the high-grade screening method for layered clay mineral according to the illustrated second embodiment of the present invention includes a primary crushing step (S210), a secondary crushing step (S220), and a primary crushing step (S230). A primary classification step (S240), a secondary pulverization step (S250), and a secondary classification step (S260) are included. At this time, the primary crushing step (S210), the secondary crushing step (S220), and the primary crushing step (S230) according to the second embodiment of the present invention are the primary crushing step and the secondary crushing of the first embodiment. Substantially the same as the step and the primary pulverization step, overlapping explanation is omitted, and the steps after the primary classification step (S240) will be described.

<Primary classification>
In the primary classification step (S240), the primary pulverized particles are subjected to wet classification using a wet classifier to be primary classified into coarse particles and fine particles . In this step, coarse particles and fine particles are determined based on a diameter of 325 mesh, particles exceeding the diameter of 325 mesh are determined as coarse particles, and particles having a diameter of 325 mesh or less are determined as fine particles and divided.

At this time, coarse particles contain a larger amount of impure minerals than fine particles .

On the other hand, in this step, of the primary classification particles, coarse particles attrition mill (attrition mill) and re-ground using a coarse particles and wet classification of the re-pulverized particles by a wet classifier and thereafter re classified to a particle, of the re-classified particles, so used in admixture with fine particles of primary classification microparticles. In this way, among the primary classified particles, coarse particles are subjected to a re-pulverization and re-classification process to collect fine particles, and then mixed with fine particles of 325 mesh or less that are separated first, There is an advantage that the quality improvement rate can be further improved.

<Secondary grinding>
In the secondary pulverization step (S250), among the primary classified particles, the fine particles are subjected to secondary pulverization using an attrition mill. In particular, in this step, the concentration of the mineral liquid and the operating conditions of the crusher may differ depending on the conditions of the raw material minerals to be charged. It is difficult to define uniformly, but the attrition mill (attrition mill) standard conditions are preferably applied. That is, it is preferable to maintain the ball charging amount of 20 to 40 v / v% of the mill internal volume, the ball size of 4 to 6 mm, and the mineral liquid concentration of 40 to 60 wt%.

<Secondary classification>
In the secondary classification step (S260), the secondary pulverized particles are wet-classified with a wet classifier and secondarily classified into coarse particles and fine particles . In this step, the fine particles can be sized to a particle size of 5 μm or less. Thus, the fine particles classified by the secondary classification are suitable for use as a fine filler.

  Meanwhile, the high-grade screening method for the layered clay mineral according to the second embodiment of the present invention may further include a third pulverization / classification step (S270) and a fourth pulverization / classification step (S280).

<Third pulverization / classification>
Referring to FIG. 3 and FIG. 4, in the third pulverization / classification step (S 270), among the particles classified secondarily, coarse particles are subjected to third pulverization using an attrition mill and then third pulverized. The particles are wet-classified by a wet classifier to be thirdarily classified into coarse particles and fine particles .

  At this time, the concentration of the mineral liquid and the operating conditions of the pulverizer can be achieved by substantially the same method as the secondary pulverization conditions. Of the particles subjected to the tertiary classification, the fine particles can be classified into particles having a diameter of 5 μm or less. As described above, the fine particles classified by the tertiary classification are suitable for use in the fine particle filler as the fine particles classified by the secondary classification.

<Fourth grinding / classification>
In the fourth pulverization / classification step (S280), coarse particles out of the third classified particles are subjected to fourth pulverization using an attrition mill, and then the fourth pulverized particles are subjected to wet classification using a wet classifier. 4th order classification into coarse and fine particles .

  At this time, the concentration of the mineral liquid and the operating conditions of the pulverizer can be achieved by substantially the same method as the secondary pulverization conditions.

On the other hand, among the quaternary classified particles, coarse particles can be classified to a diameter of 10 μm or more, and fine particles can be classified to a diameter of 10 μm or less. In this way, fine particles classified by the fourth classification are suitable for use in high-grade industrial fillers such as paint, plastic, and paper, and coarse particles classified by the fourth classification are pharmaceuticals, cosmetics, etc. Suitable for use in high-grade fillers. Therefore, the high grade sorting method of the layered clay mineral according to the second embodiment of the present invention is based on the demand of the customer or the characteristics of the raw material mineral. Classification can improve the purity of concentrate.

  The above-described high-grade screening method for layered clay mineral according to the second embodiment of the present invention has an advantage that water can be reused after the product is recovered, since the reagent is not used in the process as well as the quality improvement rate. There is.

  In addition, the high-grade sorting method of the layered clay mineral by wet grinding and classification according to the second embodiment of the present invention performs fine grain grinding for sorting, so that grinding and particle size separation for the production of the filler are completed. Since it has an advantage that it does not require a separate pulverization and classification process, it has very advantageous effects in terms of economics such as energy input and facility costs.

<Embodiment>
Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this is presented as a preferred example of the present invention and cannot be construed as limiting the present invention in any way.

  The content not described here can be technically analogized by a person skilled in this technical field, and the description thereof will be omitted.

1. Experimental Method Embodiment 1
First, a low-grade wax stone ore was primarily crushed to a size of 30 mm or less using a jaw crusher, and then secondarily crushed using a hammer crusher.

The secondary crushed particles were first pulverized using a Raymond mill in which impact force and friction force mainly act. At this time, although the pulverization particle size may show a difference depending on the thickness of the impure mineral, pulverization was performed based on 200 mesh (74 μm). The primary pulverized particles were classified into coarse particles and fine particles by wet classification based on 325 mesh (43 μm).

Thereafter, among the primary separated particles, coarse after the particles collected particulates through additional reground and re-classification process, among the primary component particle particles, attribution with particulate Tsu Deployment mill (attrition mill ) For secondary grinding. At this time, the standard conditions of the attrition mill were applied to the mineral liquid concentration and the operating conditions of the pulverizer. That is, the ball charge was maintained at 30 v / v% of the mill internal volume, the ball size was 3 mm, and the mineral solution concentration was 50 wt%.

Thereafter, the secondary pulverized particles are secondarily classified to 325 mesh using a wet classifier to classify into coarse particles and fine particles, and then the coarse particles among the secondary sized particles are attrited. After the tertiary pulverization using a mill, the tertiary pulverized particles were tertiary classified to 5 μm using a wet classifier to classify into coarse particles and fine particles .

After that, coarse particles among the pulverized particles are subjected to quaternary pulverization using an attrition mill, and then quaternized pulverized particles are subjected to quaternary classification to 10 μm using a wet classifier. It was divided into coarse particles and fine particles .

Embodiment 2
The experiment was carried out by the same method as in Embodiment 1 except that medium-grade wax stone ore was used as the ore.

2. Quality evaluation The quality of wax stone is evaluated by the content of Al ₂ O ₃ . The application may be determined based on the chemical composition, but in the field used for fine particles, the application is determined by the amount of hard mineral such as quartz, whiteness, etc.

  The fields used on the basis of chemical composition are conventional ceramic and glass fiber raw materials, etc. The fields used on the basis of particle size, particle shape, hard mineral mixing amount, etc. are existing industries such as paint and plastic. It is a raw material for cosmetics and pharmaceuticals that require high-purity products and the field of filler. Therefore, each product produced in this embodiment aims to produce existing industrial fillers and high-purity products, and the results are as follows.

  Table 1 shows the characteristics of the wax products selected according to the first embodiment.

As shown in <Table 1>, the grade of the ore was 10.78% Al ₂ O ₃ , 85.23% SiO ₂ , and the content of the dikeite mineral that can be classified into a wax stone and a wax stone mineral is It is understood that it is a relatively low grade ore at about 6.2%.

If the ore having such characteristics is pulverized and classified and separated and recovered into four types of products, the primary U / F production rate is 6.59 wt%, and the grade is 8.55% Al ₂ O ₃ , 88 It can be seen that 12% SiO ₂ is maintained. Such a phenomenon is a result of demonstrating that the quartz particles produced in a thick state with relatively high hardness are not subjected to sufficient external force and remain as coarse particles .

Since such products are mainly distributed in quartz, they can be used for existing ceramic raw materials rather than for industrial fillers. In the step of classification after pulverization using an attrition mill, the chemical component quality of the _{second and third} O / F recovered as small particles is 10.14% Al ₂ O ₃ , 86.54% SiO _2. Thus, it can be seen that the alumina quality is somewhat lower than that of the raw ore, the silica quality is improved, and the effect of improving the quality according to the selection appears.

In contrast, the quality of the chemical component of the fourth order U / F recovered after pulverization by the fourth order attrition mill is 20.14% Al ₂ O ₃ and 73.81% SiO _2. 86.83%, the silica removal rate was 13.40%, and the alumina distribution rate was 36.74%.

In addition, it is understood that the composition ratio of the rock stones is 63.4%, which is enriched by about 134% with respect to the ore, and the quality of the rock stones can be improved by the pulverization and classification methods. In the case of the 4th order O / F, the production rate was 22.98 wt%, 17.02% Al ₂ O ₃ , 77.43% SiO ₂ and lower grade than the 4th order U / F. As can be seen, it can be seen that the rocks are considerably concentrated compared to the ore.

  Such a phenomenon is considered that quartz is generally produced in a thick state, but in this deposit, it is judged that very fine quartz is partially produced. However, it is judged that it can be used for industrial fillers in fields where the lubricity is not considered as an important factor, such as cosmetics and pharmaceutical raw materials.

  Table 2 shows the characteristics of the wax products selected according to the second embodiment.

  As shown in <Table 2>, in the case of Embodiment 2, the quality improvement rate and the actual yield appeared lower than the results of Embodiment 1, but the quality improvement progressed with the same tendency in the same process. I understand that.

  At this time, in the case of the second embodiment, since the medium-grade wax was targeted, the effect of improving the quality was not clearly shown as in the result of the first embodiment. This is a result that is similar to the chemical composition.

In particular, the chemical composition grade of the fourth order U / F product, which can be called the final concentrate, is 28.78% Al ₂ O ₃ , 62.59% SiO ₂ , which is very close to the theoretical chemical composition of the rock stone. The alumina quality improvement rate was 68.37%, the distribution rate was 15.78%, the silica removal rate was 10.90%, and it could be used for the highest grade raw materials such as cosmetics and pharmaceutical raw materials. It was confirmed that possible products could be produced.

  On the other hand, the sorting techniques used in Japan and overseas have been all manual selection methods, so we performed floating screening experiments known as sorting methods applicable to improving the quality of wax stones in our laboratory. Was described.

  <Table 3> schematically shows experimental results of the experimental method, the floating selection method, and the manual selection method according to the present invention.

As shown in <Table 3>, the floating sorting method was able to obtain the most excellent results in Al ₂ O ₃ improvement rate, actual yield, SiO ₂ removal rate, and the like. On the other hand, it can be seen that the manual selection method is a conventional method that is poor in all aspects.

  On the other hand, the present invention has obtained results that are relatively excellent in terms of the quality improvement rate and the actual yield, and in particular for the purpose of improving the quality, the final product is produced by pulverization and classification. It can be seen that it has the great advantage that there is no separate grinding process for production.

  As can be seen from the above experimental results, it can be seen that the floating sorting method is considerably superior. However, floating sorting has a disadvantage that it can cause wastewater generation as well as a cause of cost increase due to the use of various reagents such as water trapping agent, foaming agent, and regulator in the process. . In particular, the floatation process collects concentrate after being pulverized to approximately 35 to 100 mesh size, so that there is a problem that a separate pulverization process must be installed for use in industrial fillers. doing. The manual selection method is a disadvantageous selection method in almost all aspects such as improvement in quality, actual yield, and labor costs.

  As described above, the embodiment of the present invention has been described mainly. However, various changes and modifications can be made at the level of an engineer having ordinary knowledge in the technical field to which the present invention belongs. Such changes and modifications can be said to belong to the present invention without departing from the scope of technical events provided by the present invention. Accordingly, the scope of the present invention should be determined by the claims set forth below.

S110 Primary crushing step S120 Secondary crushing step S130 Primary crushing step S140 Primary classification step S150 Repulverization step S160 Reclassification step S210 Primary crushing step S220 Secondary crushing step S230 Primary crushing step S240 Primary classification step S250 2 Next grinding step S260 Secondary classification step S270 Third grinding / classification step S280 Fourth grinding / classification step

Claims (12)

Primary crushing a wax or talc ore containing impure minerals including quartz or pyrite to particles with a diameter of 30 mm or less;
Secondary crushing the primary crushed particles;
Primary crushing the secondary crushed particles using a Raymond mill;
Wet-classifying the primary pulverized particles with a wet classifier to primary classification into coarse particles and fine particles ;
A high-grade sorting method for layered clay minerals, comprising: In the primary and secondary crushing steps,
The method of claim 1, wherein the primary crushing uses a jaw crusher, and the secondary crushing uses a hammer crusher. In the primary grinding step,
The method of claim 1, wherein the primary pulverization is pulverization into particles having a diameter of 200 mesh or less. In the primary classification step,
2. The high-quality sorting of layered clay mineral according to claim 1, wherein the coarse particles and fine particles are classified by classifying particles having a diameter exceeding 325 mesh as coarse particles based on a diameter of 325 mesh. Method. 5. The high-grade sorting method for layered clay mineral according to claim 4, wherein the coarse particles contain more of the impure minerals than the fine particles . After the primary classification step,
Re-grinding coarse particles among the primary classified particles using an attrition mill;
The high-grade sorting method for layered clay mineral according to claim 1, further comprising a step of wet-classifying the re-pulverized particles with a wet-classifier and re- classifying them into coarse particles and fine particles . Primary crushing a wax or talc ore containing impure minerals including quartz or pyrite using a jaw crusher to particles of 30 mm or less in diameter;
Secondary crushing the primary crushed particles using a hammer crusher;
Primary crushing the secondary crushed particles using a Raymond mill;
Wet-classifying the primary pulverized particles using a wet classifier to perform primary classification into coarse particles and fine particles ;
A step of secondary pulverizing fine particles among the primary classified particles using an attrition mill;
Wet-classifying the secondary pulverized particles with a wet classifier to secondary classification into coarse particles and fine particles ;
A high-grade sorting method for layered clay minerals, comprising: In the primary classification step,
8. The high-quality sorting of layered clay mineral according to claim 7, wherein the coarse particles and fine particles are classified by classifying particles having a diameter exceeding 325 mesh as coarse particles based on a diameter of 325 mesh. Method. In the secondary classification step,
The high-quality sorting method for layered clay mineral according to claim 7, wherein the fine particles are sized to a particle size of 5 µm or less. After the secondary classification step,
Of the secondary classified particles , a step of tertiary grinding coarse particles using an attrition mill;
Wet-classifying the tertiary pulverized particles with a wet classifier to classify them into coarse particles and fine particles ;
The high-grade sorting method for layered clay mineral according to claim 7, further comprising: After the third classification step,
A step of pulverizing coarse particles out of the tertiary classified particles using an attrition mill;
Wet-classifying the quaternary pulverized particles with a wet classifier and performing quaternary classification into coarse particles and fine particles ;
The high-grade sorting method for layered clay mineral according to claim 10, further comprising: In the primary classification step,
Among the primary classified particles, coarse particles were reground using attrition mill (attrition mill), re the re ground particles in the wet classification to coarse particles by a wet classifier and particulate The method according to claim 7, wherein fine particles are mixed with the primary classified fine particles among the reclassified particles after classification.