JP7783487B2 - Extrusion molding method - Google Patents

Description

The present invention relates to an extrusion molding method.

Sintering and fired pelleting are commonly used methods for agglomerating fine iron ore and dust generated within steelworks, but non-fired agglomeration methods, which do not require a firing process, are also used to save energy. In non-fired agglomeration methods, cement is used as a binder to give the compacts strength sufficient for use in blast furnaces. The main forming methods known are tumbling granulation and extrusion molding. In order to consistently produce compacts using the extrusion molding method, it is important to prevent the raw materials from clogging the die.

In the extrusion molding method, as shown in Figure 1, raw materials are fed into the barrel and move toward the die while being pressurized by an extrusion section (not shown), such as a screw, installed inside the barrel. The raw materials are then consolidated as they pass through the die, becoming a molded body, which is then discharged from the extruder.

The following empirical formula (1) is known to describe the pressure required to extrude the raw material in the barrel toward the die (Non-Patent Document 1).

In equation (1), P is the total pressure required to extrude the raw material, _P is the pressure loss associated with the reduction in cross-sectional area (cross-sectional area perpendicular to the longitudinal direction of the body), _P is the pressure loss associated with extrusion in the die, V is the extrusion rate of the raw material in the die, _A is the cross-sectional area of the hollow portion of the body (cross-sectional area perpendicular to the longitudinal direction of the body), A is the cross-sectional area of the hollow portion of the die (cross-sectional area perpendicular to the longitudinal direction of the body), L is the length of the hollow portion of the die, C is the perimeter of the hollow portion of the die, _τ is the yield value of the raw material, _τ is the shear stress on the inner wall surface of the body, and α and β are constants.

Powder Molding: Edited by the Society of Powder Technology, Nikkan Kogyo Shimbun, Tokyo, 2009, p. 121

When using the same extruder to maintain a constant production rate for molded products, the extrusion load fluctuates when the conditions of the raw material being molded change. The extruder's motor output (torque) is adjusted in response to fluctuations in the extrusion load, but when the extrusion load reaches the upper limit of the extruder's motor output, the extruder becomes unable to discharge the raw material (molded products) and enters a "jam" state. As a result, it becomes difficult to continue producing molded products.

Therefore, preventing clogging is important for stable production of molded products using extrusion molding, and fluctuations in the extrusion load (especially excessive increases) must be suppressed. However, no method has been proposed to date for maintaining stable production while maintaining appropriate raw material conditions and avoiding clogging.

The present invention was made in consideration of the above-mentioned problems, and its purpose is to provide a new and improved extrusion molding method that can suppress clogging of raw materials. In particular, the present invention provides an extrusion molding method that prevents clogging during extrusion molding of wet powder raw materials, with an eye toward processes for producing unsintered agglomerated ore by extrusion molding in the steel industry.

In order to solve the above problems, one aspect of the present invention provides an extrusion molding method that includes the steps of: determining the appropriate ranges of uniaxial collapse stress and shear stress of a raw material that will allow normal extrusion molding without clogging of the raw material; and determining the raw material conditions for the raw material to be used so that the uniaxial collapse stress and shear stress of the raw material to be used fall within the appropriate ranges.

Here, the raw material conditions may include at least one of the moisture content of the raw material, the particle size of the raw material, and the composition of the raw material.

The above aspects of the present invention make it possible to prevent raw material clogging, thereby preventing a decrease in productivity.

FIG. 1 is a side cross-sectional view showing an outline of an extrusion molding method. FIG. 1 is an explanatory diagram for explaining the definition of uniaxial collapse stress. FIG. 1 is a side cross-sectional view showing an example of an extrusion molding machine. FIG. 1 is an explanatory diagram showing an outline of a powder bed shear test method and a method for measuring uniaxial collapse stress. 1 is a graph showing an example of measurement results of uniaxial collapse stress and shear stress.

<1. Investigation by the Inventor>
First, the inventor's investigation will be described. According to the above-mentioned formula (1), the factors that cause fluctuations in the extrusion load during operation are the raw material properties, i.e., τ _y and τ _0. It is believed that the phenomenon that generates the extrusion load and the jamming phenomenon that is the focus of this embodiment do not occur through the same mechanism. However, the inventor suspected that the factors that govern the jamming phenomenon may be similar to τ _y and τ _0. The inventor then focused on the uniaxial collapse stress and shear stress of the raw material as parameters that correspond to τ _y and τ ₀ , respectively, and can be measured independently as raw material properties. Note that it is usually difficult to directly measure τ _y and τ ₀ in formula (1). These parameters can be indirectly determined by actually operating an actual machine with various changes in V and die dimensions.

The inventors conceived the idea that clogging could be prevented by measuring the uniaxial collapse stress and shear stress of the raw materials in advance (before the raw materials to be used are fed into the extruder) through a powder bed shear test, and then using the results to adjust the raw material's physical properties in advance. This possibility was confirmed through experiments, as described below, leading to the completion of this invention.

<2. Extrusion molding method>
Next, the extrusion molding method according to this embodiment will be described. The extrusion molding method according to this embodiment is composed of the following three processes (steps).
(Step 1) Determine the appropriate ranges of uniaxial collapse stress and shear stress of the raw material that allow normal extrusion molding without clogging of the raw material.
(Step 2) Determine the raw material conditions for the raw material to be used so that the uniaxial collapse stress and shear stress of the raw material to be used fall within appropriate ranges.
(Step 3) Extrusion molding is carried out under the raw material conditions determined in Step 2.

(2-1. Uniaxial collapse stress)
Since the yield value _τy of the raw material in equation (1) is a parameter corresponding to the stress when the raw material begins to flow, the inventors focused on uniaxial collapse stress as an index thought to be related to it. Uniaxial collapse stress is the vertical stress (σc ₎ required to collapse (flow) a powder layer whose sides are not constrained (horizontal stress _σh = 0) as shown in Figure 2. Figure 2(a) shows the state in which no vertical stress is applied to the compact, while Figure 2(b) shows the state in which a vertical stress is applied to the compact by the pressing part, causing the compact to collapse. Uniaxial collapse stress can be determined by a powder bed shear test. Details will be described later.

(2-2. Shear stress)
In actual extruders, raw materials adhere to the inner wall surface. Therefore, the shear stress _τ0 on the inner wall surface of the barrel can be considered to be the shear stress between powder particles. Therefore, the inventors focused on the shear stress measured in a powder bed shear test as an index thought to be related to the shear stress _τ0 on the inner wall surface of the barrel.

(2-3. Extrusion molding process)
The extrusion molding process to which this embodiment can be applied is not particularly limited. The raw materials for the extrusion molding process are, for example, those generally used in the production of non-calcined agglomerates, and include iron ore, coke powder, and steelmaking dust, and cement as a binder. The raw materials are usually pulverized using a ball mill or the like to obtain particles of 1 mm or less. The pulverized raw materials are adjusted to a moisture content suitable for extrusion molding and then molded using an extrusion molding machine. The obtained molded body is cured for a predetermined period of time, during which strength is developed by the hardening action of the cement hydration reaction, and the molded body becomes non-calcined agglomerates.

Here, examples of extrusion molding machines include screw-type extrusion molding machines and roller-type extrusion molding machines (Figure 3). In a screw-type extrusion molding machine, raw material fed into the barrel is pushed out of the die section by a screw rotating inside the barrel. In a roller-type extrusion molding machine, raw material fed into the barrel is pushed out of the die section by a roller rotating inside the barrel. In either case, clogging can occur depending on the properties of the raw material. The extrusion molding method according to this embodiment is applied to extrusion molding machines where clogging is a problem.

(2-4. Method for measuring uniaxial collapse stress)
The uniaxial collapse stress is one of the indicators measured by a powder bed shear test, and is the vertical stress required to collapse (flow) a powder bed whose sides are not restrained (horizontal stress σ _h = 0) as shown in Figure 2.

Main powder bed shear test devices include the Jenike cell type, rotating cell type, lower cell direct acting type, and parallel plate type, and any of these may be used in this embodiment. Details of each test device and measurement methods using powder bed shear tests are provided in JIS Z8835:2016.

Figure 4 shows an overview of the powder bed shear test method and the method for measuring uniaxial collapse stress. First, the sample is filled into the measurement cell (Figure 4(b)). The measurement cell can be divided into upper and lower halves, enabling shearing. Next, a predetermined normal stress is applied to the sample to consolidate it (pre-consolidate it). Next, the sample powder layer is sheared while applying normal stress σ, and the shear stress τ is measured. The normal stress applied to the sample powder layer is varied, and the shear stress is repeatedly measured. The failure envelope is then plotted on a σ-τ diagram (Figure 4(a)). Next, a Mohr's circle is drawn that is tangent to the failure envelope and passes through the origin (Figure 4(a)). The intersection of the Mohr's circle and the σ axis is the uniaxial collapse stress _σc .
The uniaxial collapse stress may also be measured by a uniaxial compression test in which a normal stress is applied to the molded article in a state where the side surfaces of the molded article are not restrained, as shown in FIG.

(2-5. Method for measuring shear stress)
As mentioned above, shear stress is measured in the process of determining uniaxial collapse stress. Since shear stress varies depending on the magnitude of normal stress, the shear stress at a specific normal stress is used to determine the appropriate range of uniaxial collapse stress and shear stress. Normally, shear stress varies linearly with normal stress, so there are no particular restrictions on the normal stress used. However, once the normal stress is determined, the shear stress at that normal stress is used.

(2-6. Step 1: Determining the appropriate ranges of uniaxial collapse stress and shear stress)
In step 1, the appropriate ranges of uniaxial collapse stress and shear stress of the raw material are determined so that normal extrusion molding is possible without clogging of the raw material. In other words, the uniaxial collapse stress and shear stress of the raw material are measured by the powder bed shear test described above when extrusion molding is performed normally and when clogging occurs, and the appropriate ranges of uniaxial collapse stress and shear stress so that clogging does not occur are determined. Here, clogging refers to a state in which the raw material clogs the die and the compact cannot be extruded from the die.

More specifically, raw materials were sampled at least once when extrusion molding was normal and when clogging occurred, and their uniaxial collapse stress and shear stress were measured. An example of the measurement results is shown in Figure 5 (specific measurement methods will be described in the Examples). The raw materials when clogging occurred exhibited high values of uniaxial collapse stress and shear stress. Considering the above-mentioned formula (1), this is thought to be due to an increase in τ _y (related to uniaxial collapse stress) and τ ₀ (related to shear stress), resulting in an increase in the extrusion load. On the other hand, the uniaxial collapse stress and shear stress when extrusion molding was normal exhibited values lower than those when clogging occurred. In the example of Figure 5, the appropriate ranges are a uniaxial collapse stress of 15.5 kPa or less and a shear stress of 9.6 kPa or less. Here, the upper limit of the appropriate range is the maximum value of the uniaxial collapse stress and shear stress when extrusion molding was normal. The lower limit of the appropriate range is not particularly limited, but it should be a value that allows a molded product to be extruded.

(2-7. Step 2: Determining raw material conditions)
Next, the raw material conditions for the raw material to be used are determined so that the uniaxial collapse stress and shear stress of the raw material to be used fall within appropriate ranges.

Specifically, the uniaxial collapse stress and shear stress of the raw materials to be used are measured using a powder bed shear test. If the measured uniaxial collapse stress and shear stress are within the appropriate range, the material can be used directly for extrusion molding. On the other hand, if the measured uniaxial collapse stress and shear stress are outside the appropriate range, the raw material conditions are changed and the uniaxial collapse stress and shear stress are repeatedly measured to determine conditions that bring them within the appropriate range. Specifically, for example, at least one of the moisture content, raw material particle size, and raw material composition is changed. The raw material composition refers to, for example, the types of iron ore, coke powder, and steelmaking dust, as well as the proportions of these materials.

As the moisture content of the raw material increases, liquid bridges are formed between the raw material particles, improving the bonding strength between them. This means that the uniaxial collapse stress and shear stress increase. However, when the moisture content becomes excessive and exceeds the capillary region (a state in which the voids between the raw material particles are completely filled), the raw material changes to a slurry-like state, and the uniaxial collapse stress and shear stress decrease. Extrusion molding is typically performed at moisture levels lower than the capillary region, so the moisture content is changed below the capillary region.

Reducing the particle size of the raw materials (pulverizing the raw materials) increases the adhesiveness of the raw materials, improving uniaxial collapse stress and shear stress. On the other hand, if the raw material particle size is excessively large, clogging of the die holes and jamming due to particle bite-in can occur, so the raw material particle size is preferably 1 mm or less. Here, raw material particle size is measured, for example, by laser diffraction scattering or sieving. Furthermore, raw material particle size is preferably expressed as the median diameter (50% cumulative particle diameter by mass).

When the raw material composition is changed, the uniaxial collapse stress and shear stress change in complex ways due to differences in the wettability and particle shape of the raw materials. When changing the raw material composition, in addition to changing the type and mixing ratio of iron ore, coke powder, and steelmaking dust, the uniaxial collapse stress and shear stress can be adjusted by adding binders and lubricants.

(2-8. Step 3: Extrusion molding)
In step 3, extrusion molding is performed under the raw material conditions determined in step 2. Under these raw material conditions, the uniaxial collapse stress and shear stress are within appropriate ranges, so extrusion molding can be performed without causing clogging of the raw material.

As explained above, according to this embodiment, the appropriate ranges of uniaxial collapse stress and shear stress of the raw material are determined so that normal extrusion molding can be performed without clogging of the raw material, and the raw material conditions for the raw material to be used are determined so that the uniaxial collapse stress and shear stress of the raw material to be used fall within the appropriate ranges. Therefore, extrusion molding can be performed without causing clogging of the raw material. In other words, it is possible to prevent clogging of the raw material, and ultimately to prevent a decrease in productivity.

Next, we will explain an example of this embodiment. In this example, the effects of this embodiment were confirmed by performing the following steps.

<1. Step 1: Determining the appropriate ranges of uniaxial collapse stress and shear stress>
Based on the operational results of extrusion molding using various raw materials in a roller-type extruder (Fuji Paudal Co., Ltd., Model F-5S/11-175D, die hole diameter 10 mm), the uniaxial collapse stress and shear stress of the raw materials were measured four times each when the extrusion molding was normal and when clogging occurred.

A rotating cell-type apparatus (Freeman Technology, Model FT-4, cell inner diameter 50 mm) was used for the powder bed shear test. The pre-compression was 9 kPa, and then the normal stress σ was varied to 7 kPa, 6 kPa, 5 kPa, 4 kPa, and 3 kPa, and the shear stress was measured. The uniaxial collapse stress was determined using the method described above (Figure 4(a)), and the shear stress was taken as the value at a normal stress of 7 kPa. The measurement results are shown in Figure 5. The optimum ranges were determined to be 15.5 kPa or less for the uniaxial collapse stress and 9.6 kPa or less for the shear stress. The upper limits of the optimum ranges were determined to be the maximum values of the uniaxial collapse stress and shear stress when the extrusion molding was normal.

<2. Step 2: Determining raw material conditions>
Next, the uniaxial collapse stress and shear stress were measured for blended raw material A, which was to be newly used in extrusion molding. Blended raw material A was composed of 40% by mass of iron ore powder, 35% by mass of sintered dust, 20% by mass of coke powder, and 5% by mass of high-early-strength cement (dry basis proportions), with a moisture content of 12% by mass (wet basis proportion) and a raw material particle size of 110 μm (mass cumulative median diameter). Table 1 shows the measurement results.

Both the uniaxial collapse stress and shear stress of blended raw material A were outside the appropriate ranges determined in step 1. Here, blended raw material A' was created by changing the moisture content to 13% by mass and the raw material particle size to 210 μm, and the uniaxial collapse stress and shear stress were measured again. As a result, the uniaxial collapse stress and shear stress were found to be within the appropriate ranges determined in step 1.

<3. Step 3: Extrusion Molding>
Therefore, when blended raw material A' was used for extrusion molding, a molding could be formed normally without clogging.

The above describes in detail preferred embodiments of the present invention with reference to the accompanying drawings, but the present invention is not limited to such examples. It is clear that a person with ordinary skill in the technical field to which the present invention pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims (2)

determining, from operational records , appropriate ranges of uniaxial collapse stress and shear stress of the raw material for non-sintered agglomerates, which allow normal extrusion molding without clogging of the raw material;
determining raw material conditions for the raw material to be used so that the uniaxial collapse stress and shear stress of the raw material to be used fall within the appropriate ranges. The extrusion molding method described in claim 1, characterized in that the raw material conditions include at least one of the moisture content of the raw material, the particle size of the raw material, and the composition of the raw material.