Patent Description:
In order to produce pig iron in a blast furnace, it is necessary that, first, iron ores and coke are alternately charged and packed in layers into the blast furnace, the iron ores and coke are heated by high-temperature hot air blown through tuyeres, and at the same time the iron ores are reduced and smelted by CO gas generated mainly from coke. In order to stably operate such a blast furnace, it is effective to improve permeability of gas and liquid in the furnace. For this purpose, it is essential to use coke being excellent in properties, such as strength, particle size, and strength after reaction. Above all, strength is considered to be a particularly important property.

Coke strength is usually controlled by using, as an indicator, for example, a drum strength DI (<NUM>/<NUM>) measured by the drum strength test specified in JIS K <NUM>. The coal quality that determines the drum strength includes mainly a coal rank (Ro, JIS M <NUM>) and fluidity (MF, JIS M <NUM>) (Non Patent Literature <NUM> and <NUM>).

Coal fluidity is known to decrease with time because of deterioration due to oxidation in air which is referred to as "weathering". After mined from mine, coal is transported and stored repeatedly until it is charged into a coke oven, and coal is usually placed in the air atmosphere for a long time over several weeks or more. Accordingly, it is generally difficult to avoid a decrease in the fluidity of coal due to weathering. Therefore, it has been strongly desired to develop technology for inhibiting weathering of coal.

In order to inhibit weathering of coal, it is effective to reduce contact between coal and oxygen to the smallest possible degree. Patent Literature <NUM> discloses a technique in which dry ice is circulated through perforated pipes installed at the bottom of a coal pile to perform replacement with carbon dioxide. Furthermore, Patent Literature <NUM> discloses a technique in which inert gas is blown from the bottom. Furthermore, Patent Literature <NUM> discloses a technique in which, for the purpose of suppressing diffusion of oxygen from a surface layer to the inside of a coal pile, the surface layer is subjected to coating. In addition, a method of storing coal in water, a method of storing coal in a sealed coal storage tank, a method in which a surface layer of a coal pile is compacted with heavy equipment, and the like are known (Non Patent Literature <NUM>). <NPL> discloses coal blends and a method for producing coke from said coal blend, wherein the blend comprises weathered coal. <NPL> discloses a method to monitor coal weathering, comprising the measurement of pH of coal slurry. <CIT> discloses a method for measurement of pH of coal slurry.

The techniques disclosed in Patent Literature <NUM> and Patent Literature <NUM> have problems in that it is required to introduce dedicated equipment for blowing inert gas including carbon dioxide from the bottom of a coal pile at the place where the coal pile is deposited, and costs are incurred for the gas used. In the ironmaking industry, the amount of coal used and stored in the yard is several hundred thousand tons or more. Consequently, the size of dedicated equipment increases, the cost thereof increases, and operational costs also increase. Therefore, the merit of inhibiting weathering is offset, and sufficient economic benefits are not obtained. Furthermore, in the technique disclosed in Patent Literature <NUM> in which the surface layer is subjected to coating, there are also problems in that an operation of spraying a coating material is required and the material costs are incurred. In addition, in the method of storing coal in water, the method of storing coal in a sealed coal storage tank, and the method in which a surface layer of a coal pile is compacted with heavy equipment, similarly, there are problems in that capital investment and operational costs are incurred.

The present invention has been made in view of the problems described above, and an object of the invention is to provide a method for producing a coal mixture in which it is possible to suppress a decrease in coal fluidity better than existing techniques by a simple method without excessive capital investment or operational costs.

The features of the present invention which solves these problems are as specified in the appended claims.

According to the present invention, it is possible to suppress a decrease in coal fluidity due to weathering by a very simple method of mixing a plurality of coals. Usually, facilities for mixing coals are provided in mines, call centers, loading ports, and coke plants, which handle coal, for the purpose of adjusting the amount and quality of coal products. Since the present invention can be carried out using such existing facilities, weathering of coal can be inhibited without additional capital investment for the facilities.

The present inventors have found that the speed of coal weathering varies depending on the pH (i.e., hydrogen ion concentration) of water that adheres to coal and that the amount of hydrogen ions that are dissolved in water varies depending on the types of coal, and thus have considered that by blending different types of coal and adjusting the pH of adhesion water of coal, it is possible to control the speed of coal weathering at a low level. As a result of thorough studies to verify this hypothesis, the present inventors have found optimal conditions in which it is possible to better suppress a decrease in fluidity due to coal weathering by transporting and storing coals as a coal mixture rather than by transporting and storing coals individually.

First, the effect of the pH of treatment water on the speed of coal weathering will be described. Coal was immersed in treatment waters having different pH values, and changes over time in coal fluidity were checked. The pH of treatment waters was adjusted using hydrochloric acid and pure water to a range of pH <NUM> to <NUM>. Table <NUM> shows properties of the coal used.

<FIG> is a graph showing the relationship between the reaction treatment time and the fluidity of coal. The horizontal axis of <FIG> represents the reaction treatment time (h), and the vertical axis represents the logMF (ddpm/log) of coal. As shown in <FIG>, it has been found that as the pH of treatment water decreases, a decrease in coal fluidity becomes faster, and coal weathering proceeds faster. It is known that as the pH decreases, the oxidation-reduction potential increases, and as the oxidation-reduction potential increases, an aqueous solution becomes more oxidizing. From the above result, it has been considered that as coal is treated with an aqueous solution having a lower pH, coal oxidation is promoted, and coal weathering is accelerated.

Next, a description will be made on the pH of water obtained after each of various brands of coal was immersed in a predetermined amount of water, and the hydrogen ion release capacity per unit mass of coal which is defined by the pH. Each of various brands of coal in an amount of <NUM> was immersed in <NUM> of pure water, and changes over time in the pH of water heated to <NUM> were measured. The hydrogen ion release capacity is calculated by dividing the product of hydrogen ion concentration calculated from the pH of water and the volume of the water in which the coal is immersed by the mass of the coal immersed. Table <NUM> shows the hydrogen ion release capacity of each of the brands of coal. In the case where the hydrogen ion release capacity is small, the pH of water in which coal is immersed increases to more than <NUM> since hydrogen ions are accepted from water.

<FIG> is a graph showing changes over time in the pH of various brands of coal. The horizontal axis of <FIG> represents the immersion time (min), and the vertical axis represents the pH of water in which coal is immersed. As shown in <FIG>, the pH of water in which coal was immersed varied widely from acidic to basic depending on the brands of coal. There is a possibility that this result may be caused by differences in the amounts of water-soluble sulfate minerals and the types and amounts of organic acids contained in coal. As described above, since the pH of water in which the coal was immersed widely varied depending on the brands of coal, as shown in the results of Table <NUM>, the hydrogen ion release capacity of coal widely varied depending on the brands of coal.

From the results, the present inventors have considered that by blending coals having different hydrogen ion release capacities, the pH of adhesion water adhering to the coals is controlled, and thus weathering of the coal mixture can be inhibited. That is, it has been considered that since the amount of water that adheres to coal (moisture content) during transportation and coal storage is about <NUM>% by mass, reactions by acids and bases take place among coals constituting the coal mixture via <NUM>% by mass of adhesion water, and the reactions influence the speed of coal weathering, and it has been found that by blending a plurality of coals so that the pH of adhesion water increases, it is possible to suppress a decrease in fluidity due to weathering of the coal mixture. Thus, the present invention has been made. The present invention will be described below by way of embodiments of the invention.

In a method for producing a coal mixture according to an embodiment, by blending a plurality of coals so that αcalc calculated by formula (<NUM>) below is <NUM> × <NUM>-<NUM> (mol/g-coal) or less, a coal mixture is produced. That is, a coal mixture which satisfies both formula (<NUM>) and formula (<NUM>) below is produced by blending individual brands of coal. [Formula <NUM>] <MAT>
<MAT>.

In the formula (<NUM>) and the formula (<NUM>), αcalc is the hydrogen ion release capacity per unit mass (mol/g-coal) of the coal mixture, αi is the hydrogen ion release capacity per unit mass (mol/g-coal) of a coal i, xi is the blending ratio of the coal i blended in the coal mixture, and N is the total number of brands of coal contained in the coal mixture.

Here, αi is the hydrogen ion release capacity per unit mass (mol/g-coal) of a coal i blended in the coal mixture. The hydrogen ion release capacity is calculated by measuring a pH of water in which a candidate coal to be blended in a coal mixture is immersed and dividing the product of hydrogen ion concentration calculated from the pH and the volume of the water in which the coal is immersed by the mass of the coal immersed. When the amount of water in which coal is immersed is too small, the hydrogen ion dissolution reaction does not reach equilibrium, and the hydrogen ion release capacity is calculated to be too low, which is undesirable. When the amount of water in which coal is immersed is too large, a change in hydrogen ion concentration due to immersion of coal is small, and accuracy in measurement of the hydrogen ion release capacity deteriorates, which is undesirable. Therefore, when the pH of water in which coal is immersed is measured, the mass ratio of coal to water (coal:water) is in a range of <NUM>:<NUM> or more and <NUM>:<NUM> or less.

As shown in <FIG>, the pH of water in which coal is immersed slightly changes until the dissolution reaction reaches equilibrium. Accordingly, it is preferable to measure pH after equilibrium is reached. A higher temperature of water in which coal is immersed is preferable. As the temperature of water is increased, the dissolution reaction is promoted, and the time until the dissolution reaction reaches equilibrium is shortened. Thus, pH measurement can be performed quickly. Furthermore, a longer period of time from immersion of coal in water until measuring of pH is preferable.

On the other hand, when the temperature of water in which coal is immersed is too high or the period of time until measuring of pH is too long, coal is weathered, which is undesirable. From these viewpoints, the temperature of water in which coal is immersed is set in a range of <NUM> or higher and <NUM> or lower and the period of time in which coal is immersed is set in a range of <NUM> hour or more and <NUM> hours or less. As the particle size of coal decreases, the period of time until pH reaches equilibrium decreases, but weathering is likely to proceed more quickly. Therefore, it is not necessary to dare to finely pulverize coal. Since stirring during immersion of coal can shorten the period of time until pH reaches equilibrium, stirring may be performed. However, without stirring, if coal is immersed for <NUM> hour or more, pH comes very close to the equilibrium value. Therefore, coal may be just immersed in water without stirring.

In this way, when a hydrogen ion release capacity of a candidate coal to be blended in a coal mixture can be calculated, a product of the hydrogen ion release capacity of each of coals blended in a coal mixture and the blending ratio is calculated. Types of coal and a blending ratio are determined so that the total sum of the products is <NUM> × <NUM>-<NUM> (mol/g-coal) or less. The blending ratio xi is calculated by dividing the mass of the coal i blended by the mass of the coal mixture.

For example, in the case where two coals are blended to produce a coal mixture, when one coal has a hydrogen ion release capacity of more than <NUM> × <NUM>-<NUM> (mol/g-coal), a coal having a hydrogen ion release capacity of less than <NUM> × <NUM>-<NUM> (mol/g-coal) is selected as the other coal. The blending ratio of each of the coals is determined so that the total sum of products of the hydrogen ion release capacities and the blending ratios of the coals is <NUM> × <NUM>-<NUM> (mol/g-coal) or less. By determining the types of coal to be blended in a coal mixture and the blending ratio in this way and performing blending, it is possible to produce a coal mixture in which a decrease in fluidity due to weathering is suppressed.

The coals blended may be mixed by a commonly used mixing method. Examples of the coal mixing method include a method in which mixing is performed at a transfer section of a belt conveyor, a method in which mixing is performed in a hopper, a method of mixing using heavy equipment, a method in which dedicated blending equipment, such as yard blending or blending bins, is used, and a method of mixing using a mixer. Transport and coal storage may also be performed by commonly used methods. By pulverizing a plurality of types of coal at the same time, pulverization and mixing may be combined.

As described above, the method for producing a coal mixture according to the embodiment can be carried out by only blending a plurality of coals so that αcalc calculated by the formula (<NUM>) described above is <NUM> × <NUM>-<NUM> (mol/g-coal) or less and, therefore, can be carried out by a simple method without excessive capital investment or operational costs. Furthermore, by charging the coal mixture in which a decrease in coal fluidity is suppressed into a carbonization chamber of a coke oven and performing carbonization, coke having high strength can be produced.

As the transportation and coal storage time increases, the degree of decrease in fluidity due to weathering increases. Accordingly, it is preferable to carry out the method for producing a coal mixture according to the embodiment as early as possible after coal is mined, and it is preferable to carry out the method at least before carrying the coal into a coke plant equipped with a coke oven. Thus, the effect of suppressing a decrease in fluidity can be increased.

Next, a description will be made on results of evaluation of coal mixtures produced by a method for producing a coal mixture according to the embodiment. Using a thermostat for the purpose of adjusting weathering conditions, changes in fluidity of coal mixtures were checked in the case where two brands of coal were blended and stored as a coal mixture in the thermostat (before thermostat treatment) and in the case where the same two brands of coal were separately stored in the thermostat and then blended together (after thermostat treatment). The properties, pH, and hydrogen ion release capacity of coal used are shown in Table <NUM>. Coal (<NUM>) was immersed in <NUM> of pure water maintained at <NUM>, and after the coal was immersed for <NUM> hours in the water, the hydrogen ion release capacity of the coal was calculated from the pH of the water.

Each of the brands of coal shown in Table <NUM> was pulverized to a particle size of <NUM> or less. Two brands of coal were blended so that the mass ratio on dry basis was <NUM>:<NUM> to produce a coal mixture, and the moisture content was adjusted to <NUM>% by mass. The coal mixture was packed in a closed container, and the closed container was stored in a thermostat kept at <NUM> for <NUM> weeks. Then, the fluidity of the coal mixture was measured.

On the other hand, each of the same brands of coal as above was pulverized to a particle size of <NUM> or less, and the coal whose moisture content was adjusted to <NUM>% by mass was packed in a closed container, and the closed container was stored in a thermostat kept at <NUM> for <NUM> weeks. Then, two brands of coal after storage were blended so that the mass ratio on dry basis was <NUM>:<NUM> to produce a coal mixture. The fluidity of the coal mixture was measured. The results thereof are shown in Table <NUM>.

The value under the column "Hydrogen ion release capacity" of Table <NUM> is the hydrogen ion release capacity per unit mass of the coal mixture (αcalc) calculated using the formula (<NUM>) described above. For example, in the case of Level No. <NUM> of Table <NUM>, the calculation was made by [hydrogen ion release capacity of Coal e (<NUM> × <NUM>-<NUM>) × blending ratio (<NUM>)] + [hydrogen ion release capacity of Coal c (<NUM> × <NUM>-<NUM>) × blending ratio (<NUM>) ].

The value under the column "Before thermostat treatment" is the measured value of fluidity of the coal mixture which was produced by blending two brands of coal before storage in the thermostat and then storing in the thermostat. The value under the column "After thermostat treatment" is the measured value of fluidity of the coal mixture which was produced by storing the same two brands of coal as above separately in the thermostat and blending the coals after storing. The value under the column "Before treatment - After treatment" is the difference between the measured value "before thermostat treatment" and the measured value "after thermostat treatment".

<FIG> is a graph showing the relationship between the hydrogen ion release capacity of coal mixture and the fluidity "before treatment - after treatment". The horizontal axis of <FIG> represents the hydrogen ion release capacity of coal mixture (mol/g-coal), and the vertical axis represents the fluidity "before treatment - after treatment" (ddpm/log). Here, a positive value of fluidity "before treatment - after treatment" indicates that a decrease in fluidity is small when coals are stored as a coal mixture in the thermostat compared with the case where coals are stored separately as individual coals in the thermostat. On the other hand, a negative value of fluidity "before treatment - after treatment" indicates that a decrease in fluidity is large when coals are stored as a coal mixture in the thermostat compared with the case where coals are stored separately as individual coals in the thermostat.

Claim 1:
A method for producing a coal mixture comprising blending a plurality of coals, wherein formula (<NUM>) and formula (<NUM>) below are satisfied:
[Formula <NUM>] <MAT> <MAT>
where, in the formula (<NUM>) and the formula (<NUM>), αcalc is the hydrogen ion release capacity per unit mass (mol/g-coal) of the coal mixture,
αi is the hydrogen ion release capacity per unit mass (mol/g-coal) of a coal i,
xi is the blending ratio of the coal i blended in the coal mixture, and
N is the total number of brands of coal contained in the coal mixture, wherein the hydrogen ion release capacity per unit mass of coal is calculated by dividing the product of hydrogen ion concentration calculated from pH of water in which each of the coals is immersed and the volume of the water in which the coal is immersed by the mass of each of the corresponding coals, wherein the temperature of water in which coal is immersed is in a range of <NUM> or higher and <NUM> or lower and the period of time in which coal is immersed is in a range of <NUM> hour or more and <NUM> hours or less, wherein the mass ratio of coal to water is in a range of <NUM>:<NUM> or more and <NUM>:<NUM> or less, and wherein the hydrogen ion release capacity is further measured by utilizing the method described in the description.