Patent Description:
Metallurgical coke used as blast furnace feed material to produce molten iron in blast furnaces is preferred to have high strength. This is because coke with low strength is degraded in a blast furnace to reduce the gas permeability of the blast furnace and therefore molten iron cannot be stably produced. Thus, a technique for evaluating coal as a raw material for metallurgical coke is required from the viewpoint of obtaining high strength coke or the viewpoint of not reducing the strength of coke.

Patent Literature <NUM> describes that coal in a plastic state has a significant influence on the quality of coke during coking process in a coke oven. As described above, in the evaluation of coal, it is important to precisely evaluate properties of coal in a plastic state. As described in Patent Literature <NUM>, a fluidity measurement method using a Gieseler plastometer method specified in JIS-M8801 is known as a method for evaluating the same.

Patent Literature <NUM> discloses a coal mixture in which a coal blend is composed with which it is intended to be possible to maintain high level of coke strength after carbonization of the coal blend even if coal having an excessively large permeation distance is used in a large amount.

Patent Literature <NUM> discloses obtaining coke by carbonizing a blended coal which is composed of a plurality of brands of coal and which exhibits properties within a total inert content range of <NUM>-<NUM> vol. % and a maximum fluidity (log MF) range of <NUM>-<NUM> log ddpm according to a Gieseler Plastometer, wherein the ratio of the total cross-sectional area of pores having a circularity of <NUM> or more among large pores having diameters of <NUM> to <NUM> in the coke to the total cross-sectional area of the large pores is <NUM>% or more.

Non-Patent Literature <NUM> discloses a study of the standardization of the Giseler Plastometer and the behavior of coals in the plastic stage using standard substance and retort made of acrylonitoril resin or quartz-glass.

As described in Patent Literature <NUM>, it is known that there is a problem in that it is uncertain whether the use of fluidity measured with a Gieseler plastometer simulates a phenomenon occurring in an actual coke oven. There is a problem in that estimating the quality of coke using the fluidity of coal measured with a Gieseler plastometer as an index is not sufficient in terms of accuracy. A technique for evaluating coal as a raw material for metallurgical coke using an index other than the fluidity of coal is required.

The present invention is intended to solve the above problems and has an object to provide a method for evaluating whether there is a probability that coal intended to be evaluated reduces the strength of coke using an apparatus, such as a Gieseler plastometer hitherto widely known, including a container storing coal and a stirrer insertably placed in the container. Furthermore, the present invention has an object to provide a method for preparing a coal blend containing coal evaluated by the method and a method for producing coke by carbonizing a coal blend obtained by the preparing method.

In an experiment performed to measure the Gieseler fluidity, the inventors have observed a phenomenon that the shape of heated coal (semicoke) remaining in a container of a Gieseler plastometer after measurement varies depending on coals. The inventors have investigated whether this shape can be used to evaluate coal, leading to the completion of the present invention. That is, the present invention is as summarized below.

According to the present invention, whether there is a probability that coal intended to be evaluated reduces the strength of coke can be grasped. Even in the case of using coal rated poor in the present invention in a coal blend used as a source of coke, if the mass percentage of the coal in the coal blend, the mass percentage being capable of suppressing the reduction in strength of coke, is grasped, then an operation producing coke in such a manner that the reduction in strength of coke is suppressed and the usage of the coal is optimized can be achieved. This enables coal previously regarded as unusable to be used. Furthermore, even in the case of using coal rated poor in the present invention in a coal blend, coal forming a coal blend capable of producing coke with desired strength and the mass percentage thereof can be identified.

The present invention provides a method for evaluating coal using the shape of semicoke formed from coal heated with an apparatus including a container storing coal and a stirrer insertably placed in the container as an index. In particular, the method is such that the degree of entanglement (a - b)/a represented by the height b of semicoke on an inner wall of the container and the height a of the semicoke on the stirrer or the height a only is used as an evaluation index for coal.

<FIG> is a vertical sectional view showing an example of a Gieseler plastometer <NUM> usable in this embodiment. The Gieseler plastometer <NUM> includes a container <NUM> storing coal intended to be evaluated and a stirrer <NUM> insertably placed in the container <NUM>. The stirrer <NUM> is equipped with a driving device, which is not shown, and is rotatable. The driving device applies predetermined torque to the stirrer <NUM> in such a state that the stirrer <NUM> is inserted in the coal stored in the container <NUM>. Next, heating the container <NUM> allows the heated coal <NUM> to be in a plastic state. Since the coal <NUM> is a viscoelastic body, the coal <NUM> is deformed and is entangled with the rotating stirrer <NUM>. Force to maintain a shape works on the coal <NUM> and force to resist rotation exerts on the stirrer <NUM>.

In a fluidity measurement method using a Gieseler plastometer method, the rotational speed of the stirrer <NUM> is measured in such a state that predetermined torque is applied to the stirrer <NUM>, followed by determining the maximum rotational speed during heating as the Gieseler maximum fluidity MF (ddpm). In some cases, a measurement value is represented by log MF, which is the common logarithm of the Gieseler maximum fluidity MF. Measurement conditions such as the heating temperature of coal and the size of the container <NUM> are specified in JIS M <NUM> and are as described below.

The stirrer <NUM>, which is equipped with a shaft with a diameter of <NUM> and four crossbars (a diameter of <NUM> and a length of <NUM>) perpendicular to the shaft, is inserted into the container <NUM>, which has a depth of <NUM> and an inside diameter of <NUM>, followed by filling the container with <NUM> of coal. Next, the container <NUM> is dipped in molten metal preheated to <NUM> or <NUM> and heating at a rate of <NUM>/minute is continued until the rotation of the stirrer <NUM> stops. Herein, the distance between the lowest crossbar of the stirrer <NUM> and the bottom of the container is <NUM> and the distance between the crossbars in an axial direction is <NUM>. The central two crossbars are located at positions <NUM> degrees different from each other, the uppermost and lowermost crossbars are also located at positions <NUM> degrees different from each other, and the central two crossbars and the two uppermost and lowermost crossbars are located at positions <NUM> degrees different from each other. Conditions specified in ASTM D2639 are similar to conditions specified in JIS M <NUM> and therefore a method of ASTM may be used. In the case not using a Gieseler plastometer, a stirrer with a diameter that is <NUM>% to <NUM>% of the inside diameter of a container storing coal is preferably used. The stirrer is preferably equipped with crossbars. Even if the stirrer is equipped with no crossbars, the entanglement of softened or molten coal with the stirrer occurs.

Coal is softened and melted by heating to exhibit fluidity and molten coal is resolidified by further heating. Therefore, after measurement under the above-mentioned conditions, coal heated under conditions that the heating temperature is higher than or equal to the resolidification temperature of the coal is converted into semicoke <NUM>, which is stored in the container <NUM>. Coal and semicoke are plastic bodies. Therefore, after the Gieseler fluidity is measured, the coal (semicoke) <NUM> in heating and stirring is in contact with an inner wall of the container <NUM>, is pulled with the stirrer <NUM>, and is held in such a form that the coal (semicoke) <NUM> is entangled with the stirrer <NUM>. Thus, in most brands of coal, as shown in <FIG>, the height a of the semicoke <NUM> in contact with the stirrer <NUM> from the bottom of the container <NUM> is greatest and the height b of the semicoke <NUM> in contact with the inner wall of the container <NUM> from the bottom is least. The behavior of softened or molten coal is known as the Weissenberg effect.

The heights a and b can be measured by disassembling the container after measurement. An image of the shape of semicoke can be obtained by scanning the container <NUM> with a microfocus X-ray CT system after the measurement of fluidity. The heights a and b can be measured from the image. The microfocus X-ray CT system is, for example, XTH320LC manufactured by Nikon Corporation, phoenix v | tome | x m300 manufactured by GE Sensing & Inspection Technologies Co. , or the like. Since there is little difference depending to a position in a circumferential direction of the container for the height a and the height b, it is usually sufficient to measure the shape of a cross section. If there is a difference depending to a position therebetween, the height is measured in a plurality of cross sections and the average of the measurements may be used as the value of the height a or b.

The shape of semicoke after the measurement of Gieseler fluidity varies depending on coal. The inventors have conceived that the height of semicoke in a container serves as an index showing the influence on the strength of coke, have investigated the relationship between the degree of entanglement (a - b)/a represented by the height of semicoke in a container and the strength of coke, and have found that the strength of coke obtained from the coal can be estimated from the degree of entanglement. The inventors have found that even if the height a of semicoke on a stirrer is used instead of the degree of entanglement, the strength of coke can be estimated as is the case with the degree of entanglement.

In a plastic state, coal with a high degree of entanglement and coal in which the height a of semicoke on a stirrer is large have excessively high dilatation, are likely to cause a defect structure in heated coke, and are supposed to have a negative influence on the strength of coke. Thus, in this embodiment, when the degree of entanglement or height a of coal is greater than or equal to a predetermined value, the coal is evaluated as poor. For example, under measurement conditions of a Gieseler plastometer specified in JIS or the like, coal with a degree of entanglement of <NUM> or more or coal with a height a of <NUM> or more is rated poor as coal for metallurgical coke. As the degree of entanglement and the height a are larger, the dilatation is higher, which can be judged to have a negative influence on the strength of coke. Therefore, for the degree of entanglement and the height a, no upper limit for evaluating coal needs to be set. Incidentally, for both the degree of entanglement and the height a, measurement values are limited by the size of a container storing sample coal. Thus, measurement is preferably performed using a container capable of measuring a degree of entanglement of <NUM> or more or a height a of <NUM> or more.

The semicoke <NUM> is not at all in contact with the inner wall (side wall) of the container <NUM> depending on the brand of coal in some cases because all the semicoke <NUM> is pulled by the stirrer <NUM>. Even in this case, coal is supposed to have excessively high dilatation; hence, there is no harm in evaluating coal by calculating the degree of entanglement and the degree of entanglement may be calculated to be <NUM> by substituting <NUM> for b.

In an operation preparing a coal blend by mixing coal rated poor with coal different from the coal, the reduction in strength of coke produced by carbonizing the coal blend can be suppressed by suppressing the mass percentage of the coal rated poor in the coal blend. In this embodiment, the coal blend is prepared such that the mass percentage of the coal rated poor in the coal blend is, for example, <NUM>% by mass or less. This enables the reduction in strength of coke to be suppressed in most operations.

Upon performing an operation, a plurality of coal blends differing in the mass percentages of coal rated poor and coal different from the coal are prepared and the relationship between the strength of coke obtained by carbonizing each coal blend and the mass percentage of coal rated poor is obtained in advance. This enables the mass percentage of the coal rated poor to be identified from the relationship therebetween such that the strength of coke is greater than or equal to a desired value in the operation and allows a coal blend to be prepared such that the mass percentage of the coal rated poor in the coal blend is less than or equal to the identified mass percentage. As a result, a coal blend can be prepared using coal rated poor such that the strength of coke is greater than or equal to a desired degree.

A coal blend may be prepared in such a manner that the relationship between the strength of coke and the mass percentage of coal rated poor is obtained in advance and the mass percentage of the coal rated poor is identified from the relationship obtained in advance such that the strength of coke is greater than or equal to a desired value. That is, an entity that prepares a coal blend may be different from an entity that obtains the relationship. Herein, the term "entity" refers to a person or organization that performs the act. Coke with a strength greater than or equal to a desired value can be produced in such a manner that coke is produced by carbonizing a coal blend prepared as described above in a coke oven or the like.

Next, the following experiments are described: experiments in which various coals with different properties were prepared and in which correlations between the height a of semicoke on a stirrer, the height b of semicoke on an inner wall of a container, the degree of entanglement (a - b)/a, and the Gieseler maximum fluidity log MF were investigated. <FIG> includes graphs showing correlations between the height a of semicoke on a stirrer of a Gieseler plastometer, the height b of semicoke on an inner wall of a container, the degree of entanglement (a - b)/a, and the Gieseler maximum fluidity log MF. <FIG> is a graph showing the relationship between the height a on the stirrer and log MF. <FIG> is a graph showing the relationship between the height b on the inner wall of the container and log MF. <FIG> is a graph showing the relationship between the degree of entanglement (a - b)/a and log MF.

According to the graph of <FIG>, the height a increases with the increase of log MF and this can be read as if a positive relationship holds between log MF and the height a. However, as indicated by enclosure in ○ in the graph, points that differ in the value of a even though log MF is almost the same, about <NUM>, are confirmed. Thus, it is hard to say that a positive relationship holds between log MF and the height a.

According to the graph of <FIG>, data varies and it cannot be read that a relationship holds between log MF and the height b. As is the case with a in <FIG>, a plurality of points that are almost identical in log MF and that differ in the value of b are confirmed. Thus, it cannot be said that a relationship holds between log MF and the height b.

As indicated by rectangular enclosure in the graph of <FIG>, two points that differ in log MF and that are identical in the degree of entanglement, which is <NUM>, are confirmed. As indicated by circular enclosure in the graph, the degree of entanglement differs even though log MF is almost the same. From these results, it cannot be said that a relationship holds between log MF and the degree of entanglement.

In view of the above results, it cannot be said that the degree of entanglement, which is an evaluation index used in this embodiment, correlates with the Gieseler maximum fluidity and it can be said that the degree of entanglement is an evaluation index different from the Gieseler maximum fluidity.

Black square plots in <FIG> represent two types of coal in which the degree of entanglement (a - b)/a is <NUM> or more. It was recognized that the two types of coal had a height a of <NUM> or more and coal with a high degree of entanglement tended to have a large height a.

In order to investigate the influence of the degree of entanglement (a - b)/a and the height a on the strength of coke, a carbonization test was performed using Coals A to F. Properties of the coals used are shown in Table <NUM>. The carbonization test was such that coke was produced in such a manner that an electric furnace capable of simulating carbonization conditions of a coke oven was used and a coal blend charged into the furnace at a bulk density of coal charge of <NUM>/dry-coal was carbonized at <NUM>,<NUM> for six hours. Properties and the degree of entanglement (a - b)/a of the prepared coals are shown in Table <NUM>.

In Table <NUM>, "Ash" and "Volatile matter" are values (mass percent on a dry basis) measured by a method for proximate analysis in JIS M <NUM>. "Ro" is the mean maximum reflectance of vitrinite of coal in JIS M <NUM> and "TI" is the total inert (volume percent) in coal maceral analysis as calculated on the basis of Parr's formula described in a method for measuring coal macerals in JIS M <NUM> and an explanation thereof. "log MF" is the value of the common logarithm log of the maximum fluidity MF measured by a fluidity measurement method using a Gieseler plastometer method specified in JIS M <NUM>. As shown in Table <NUM>, Coals A to F have different properties.

In Table <NUM>, "Degree of entanglement" is the value of the degree of entanglement (a - b)/a calculated using the heights a and b measured by a method for evaluating coal according to this embodiment using the Gieseler plastometer shown in <FIG>. The heights a and b were actually measured from an image of the cross-sectional shape of semicoke that was obtained by scanning the container <NUM> with an X-ray CT system, XTH320LC, manufactured by Nikon Corporation.

What is noteworthy in Table <NUM> is that Coals A and B have a height a of <NUM> or more and a degree of entanglement of <NUM> or more. Coal F can be regarded as standard coal in the technical field of producing metallurgical coke from coal in view of properties such as Ro and log MF shown in Table <NUM>.

In this example, furthermore, coke was produced by carbonizing a coal mixture, composed of two types of coal, obtained by mixing each of Coals A to E with Coal F at a ratio of <NUM>:<NUM>. The strength of obtained coke is shown in Table <NUM>.

As the strength of coke, the drum strength DI <NUM>/<NUM>, which is mass ratio × <NUM>, the mass ratio being a ratio of the mass of coke with a particle size of <NUM> or more after rotation to the mass of coke before rotation, was determined in such a manner that the mass percentage of coke with a particle size of <NUM> or more was measured after a drum tester charged with a predetermined amount of coke was rotated at <NUM> rpm <NUM> times on the basis of a drum strength test method of JIS K <NUM>. In Table <NUM>, the strength of coke obtained from a coal mixture composed of two types of coal is described.

As is clear from Table <NUM>, coke obtained from a coal mixture of Coal A or B and Coal F has strength lower than that of the case of mixing Coals C, D, and E with Coal F. Coals A and B both have a degree of entanglement (a - b)/a of <NUM> or more or a height a of <NUM> or more. This allows coal with a degree of entanglement (a - b)/a of <NUM> or more to be rated poor as coking coal for cokemaking. Likewise, coal with a height a of <NUM> or more can be rated poor as coking coal for cokemaking.

Next, the limit of the blending ratio of coal rated poor as coking coal for cokemaking was investigated.

A coal mixture of Coals A and C and a plurality of brands of coal was prepared and five types of coal blends were prepared by varying the blending ratios of Coals A and C such that the blending ratio of the coal mixture was <NUM>% by mass and the sum of the blending ratios of Coals A and C was <NUM>% by mass. Coke was produced in such a manner that an electric furnace capable of simulating carbonization conditions of a coke oven was used and the coal blends were charged into the furnace at a bulk density of coal charge of <NUM>/dry basis and were carbonized at <NUM>,<NUM> for six hours. Properties of the prepared coals and coal mixture are shown in Table <NUM>. Herein, for the ash, volatile matter, Ro, TI, and log MF of the coal mixture, the average properties are shown and, for the height a and degree of entanglement thereof, values actually measured using a Gieseler plastometer are shown.

<FIG> is a graph showing the relationship between the strength DI (<NUM>/<NUM>) of coke and the mass percentages of Coals A and C in each coal blend used as a source of coke. The blending ratios of Coals A and C are clear from the mass percentage plotted in <FIG>. According to <FIG>, although Coals A and C have relatively similar properties, the strength of coke in the case of blending <NUM>% by mass of Coal A is lower than the strength of coke in the case of blending <NUM>% by mass of Coal C. That is, it can be confirmed from this test that Coal A is poor as coal for metallurgical coke.

From the graph of <FIG>, for the mass percentage of Coal A, which is rated poor, and the strength of coke, a correlation that the reduction in mass percentage of Coal A increases the strength of coke can be read. That is, suppressing the mass percentage of Coal A allows the strength of coke to be maintained at a high level. Furthermore, from the graph of <FIG>, it is clear that suppressing the mass percentage of Coal A in a coal blend to <NUM>% by mass or less enables the reduction in strength of coke to be suppressed and enables the strength of coke to be maintained at a high level. The negative influence of coal rated poor by the method for evaluating coal according to this embodiment on the strength of coke is smaller as the blending ratio thereof is lower. Therefore, the lower limit of the blending ratio of coal rated poor is <NUM>% by mass.

If the desired strength of coke is set to about <NUM> in terms of the drum strength DI (<NUM>/<NUM>), it can be identified from the graph of <FIG> that the mass percentage of Coal A with which the strength of coke can be maintained at a high level is <NUM>% by mass or less. Thus, the production of coke with desired strength can be achieved in such a manner that a coal blend is prepared such that the mass percentage of Coal A is <NUM>% by mass or less, followed by producing coke.

In this example, the following relationship is obtained: the relationship between the strength of coke obtained by carbonizing a plurality of coal blends differing in the mass percentages of coal (which is hereinafter referred to as "poor coal" and is Coal A in this example) rated poor in terms of (a - b)/a or the height a and coal different from the poor coal and the mass percentage of the poor coal. In this example, an example of the following method is shown: a method for preparing a coal blend such that the mass percentage of poor coal in which the strength of coke is greater than or equal to a desired value is identified on the basis of the above relationship and the mass percentage of the poor coal is less than or equal to the identified mass percentage.

Claim 1:
A method for evaluating coal, comprising using an apparatus including a container (<NUM>) storing coal and a stirrer (<NUM>) insertably placed in the container (<NUM>),
wherein a degree of entanglement (a - b)/a represented by a height b of semicoke (<NUM>) on an inner wall of the container (<NUM>), the semicoke being formed in the container in such a manner that the stirrer (<NUM>) is rotated while the coal stored in the container (<NUM>) is being heated to a heating temperature higher than or equal to a resolidification temperature of the coal, and a height a of the semicoke (<NUM>) on the stirrer (<NUM>) is used as an evaluation index for evaluating whether there is a probability that the coal intended to be evaluated reduces a strength of coke obtained from a coal blend containing the coal.