Patent Description:
Sintered ore is usually produced through the following steps. First, auxiliary raw material powder such as limestone, silica stone, or serpentine, miscellaneous raw material powder such as dust, scale, or return ore, and solid fuel such as coke breeze are mixed in appropriate amounts into powdery iron ore of multiple types of brands (e.g., powdery iron ore called sinter feed of up to about <NUM>) to prepare compounded raw material for sintering. Next, a necessary amount of moisture is added to the obtained compounded raw material for sintering, and the compounded raw material for sintering with the moisture added thereto is mixed and granulated using a granulator, such as a drum mixer, to obtain granulated raw material for sintering. Then, the obtained granulated raw material for sintering is charged into a sintering machine and sintered to produce sintered ore.

Generally, the component materials of the compounded raw material for sintering each contain a predetermined amount of moisture, so that, when granulated, the component materials aggregate and turn into quasi-particles. The granulated raw material for sintering resulting from turning into quasi-particles serves to secure favorable gas permeability of a sintering raw material charged layer that is formed when the raw material is charged onto the pallet of the sintering machine, and thus is effective for the smooth progress of the sintering reaction.

In the method for producing sintered ore described above, when moisture is added unevenly or in an insufficient amount during granulation of the compounded raw material for sintering, only fine powder with a small particle diameter may aggregate to form coarse particles with low strength, or fine powder with a small particle diameter may remain as particles with a small particle diameter, which causes a decrease in the gas permeability inside the sintering raw material charged layer. Under these circumstances, studies focusing on how to add water to compounded raw material for sintering during granulation have been hitherto conducted.

For example, in Patent Literature <NUM>, a water sprinkling flow rate, a spraying angle, a water sprinkling distance, etc. are specified according to the change of a surface of compounded raw material for sintering inside a granulator, and also an upper limit of the droplet diameter of water to be added is specified, to prevent the generation of coarse particles and mitigate the generation of aggregated particles composed only of fine powder.

However, the technology proposed in Patent Literature <NUM> faces a problem in that the gas permeability of the sintering raw material charged layer decreases, as it can mitigate the generation of coarse particles with a large particle diameter but cannot mitigate the generation of fine particles with a small particle diameter. As a result, many fine particles with an extremely small particle diameter are generated, so that the gas permeability inside the sintering raw material charged layer and consequently inside a sintered layer decreases and the production rate of sintered ore decreases. Moreover, the strength of sintered ore decreases and the product yield decreases, and consequently the gas permeability inside a blast furnace that uses the sintered ore decreases.

In view of these problems, the present inventors have vigorously explored a method that produces desirable granulated raw material for sintering and can consequently achieve higher productivity of sintered ore etc. by setting an optimal range of the droplet diameter of moisture added during granulation for improving the gas permeability inside the sintering raw material charged layer, instead of focusing on generation of coarse particles or fine particles.

An object of the present invention is to propose a method that increases the gas permeability of a sintering raw material charged layer on a sintering machine pallet and achieves higher productivity and improved strength of sintered ore by focusing on moisture, i.e., water, added into compounded raw material for sintering and adjusting the mean droplet diameter of the moisture to be within an optimal range so as to efficiently disperse the moisture into the compounded raw material for sintering in a granulator and thus obtain desirable granulated raw material for sintering.

The method of the present invention developed to overcome the above-described problem of the related art and achieve the object is a method for producing sintered ore that obtains sintered ore by granulating compounded raw material for sintering including iron ore powder of multiple types of brands using a granulator and sintering obtained granulated raw material for sintering by a sintering machine, wherein not less than 80mass% of moisture to be added during granulation of the compounded raw material for sintering is supplied at a mean droplet diameter of not less than <NUM> and not more than <NUM>, characterized in that the moisture contains fine particles with a particle diameter of not more than <NUM> at a ratio of not less than <NUM> mass% and not more than <NUM> mass% relative to the compounded raw material for sintering.

The present invention according to the above configuration would be able to provide more preferable solutions when the method further has the following characteristics.

The method of the present invention according to the above-described configuration contributes greatly to mitigating the generation of fine particles in granulating compounded raw material for sintering by a granulator, as well as improves not only the gas permeability of granulated raw material for sintering themselves but also the gas permeability of a raw material charged layer on a sintering machine pallet. As a result, the method achieves not only higher productivity of sintered ore and improved strength of sintered ore but also higher gas permeability inside a blast furnace.

The present inventers have studied the moisture that is used when granulating granulated raw material for sintering, particularly the relationship between the droplet diameter of the water and the gas permeability of the produced granulated raw material for sintering by varying the droplet diameter of the water. While a water spraying nozzle generally has a specific droplet diameter (a droplet diameter at a predetermined pressure indicated by the manufacturer), it is desirable that the droplet diameter of water to be added is actually measured beforehand according to the granulator before the moisture is added into the granulator.

To measure the droplet diameter, a method of performing an image analysis of not less than <NUM> droplets using a high-speed camera and calculating the arithmetic mean diameter is adopted. For measurement of the droplet diameter, for example, counting and calculation may be automatically performed by the method described in Non-Patent Literature <NUM> or by means of a commercially available laser Doppler measurement instrument, or the droplet diameter may be obtained by calculation based on the liquid to be used and spraying conditions.

It has been found that, in actual water spraying, about <NUM> mass% falls within a range of the mean droplet diameter × <NUM>% to the mean droplet diameter × <NUM>%, but that droplets such as coarse droplets that fall after collecting around a spray nozzle outlet do not fall within the range of the mean droplet diameter × <NUM>% to the mean droplet diameter × <NUM>%. However, the study by the present inventers has also found that, even when such droplets that do not fall within the range of the mean droplet diameter × <NUM>% to the mean droplet diameter × <NUM>% are generated, they have little impact on the granulation action if the ratio thereof is less than <NUM> mass%. In the present invention, therefore, it is necessary to adjust the size of water to be added during granulation such that not less than <NUM> mass% thereof has a predetermined droplet diameter.

In actual water spraying, other than the coarse droplets that fall after collecting around the spray nozzle outlet, droplets that are significantly coarser than the mean droplet diameter may be generated due to factors including deterioration of the nozzle and addition of an ultrafine powdery raw material to water. Such significantly coarse droplets prevent the present invention from producing its effects. Moreover, these droplets, even when small in number, each have a significantly large volume; as such, in adjusting the granulated raw material for sintering to a predetermined moisture content, they reduce the amount of water with a preferred droplet diameter and conversely lessen the workings and effects. To correctly evaluate the influence of such significantly coarse droplets, it is desirable to use, as the mean droplet diameter, a droplet diameter that gives an arithmetic mean of droplet volumes or a Sauter mean average, instead of the arithmetic mean diameter. The droplet diameter that gives the arithmetic mean of droplet volumes can be obtained by the following Formulas (a) and (b), and the Sauter mean diameter can be obtained by the following Formula (c). The droplet diameter that gives the arithmetic mean of droplet volumes and the Sauter mean average assume a value larger than the arithmetic mean diameter when droplets with a significantly large droplet diameter increase in a droplet diameter distribution, which makes them suitable for correctly evaluating the influence of significantly coarse droplets. <MAT> <MAT> <MAT>.

In these Formulas, Va is the arithmetic mean (m<NUM>) of droplet volumes, v is the volume (m<NUM>) of each droplet, n is the number of droplets, Da is the droplet diameter (m) that gives the arithmetic mean of droplet volumes, Dz is the Sauter mean diameter (m), and d is the droplet diameter (m) of each droplet.

When using the droplet diameter that gives the arithmetic mean of droplet volumes or the Sauter mean diameter as the mean droplet diameter, it is desirable that not less than <NUM> mass% falls within a range of the mean droplet diameter × <NUM>% to the mean droplet diameter × <NUM>%. Effective measures to achieve this include not using a deteriorated nozzle and appropriately managing the amount of ultrafine powdery raw material to be added to water.

Next, the gas permeability of granulated matter (granulated raw material for sintering) produced by a granulator, such as a drum mixer, can be evaluated by measuring a gas permeability index (JPU). In the present invention, the term gas permeability index (JPU) refers to a gas permeability index measured by suctioning atmospheric air in a downward direction, as it is, through a sintering raw material charged layer that is formed as quasi-particles are charged onto a pallet of a sintering machine. This gas permeability index (JPU) was calculated using the following Formula (<NUM>).

Here, V is an air volume (Nm<NUM>/min), S is the cross-sectional area (m<NUM>) of the raw material charged layer, h is the height (mm) of the charged raw material, and ΔP is a pressure loss (mmH<NUM>O).

It is generally known that the gas permeability index (JPU) expressed by the above Formula (<NUM>) assumes a large value when the gas permeability of the sintering raw material charged layer is high, and that the gas permeability index (JPU) expressed by the above Formula (<NUM>) assumes a small value when the gas permeability of the sintering raw material charged layer is low.

To test this gas permeability, the equipment shown in <FIG> was used. In this test, sintering raw material was kneaded for three minutes by a concrete mixer <NUM> in advance, and then was granulated with moisture added for five minutes by a drum mixer <NUM> that is a granulator. Thereafter, raw material <NUM> having been granulated (granulated raw material for sintering) was charged into a test pot <NUM> having a diameter of <NUM> so as to have a raw material layer thickness H of <NUM> (raw material weight: <NUM>). Then, air was suctioned by a suction blower <NUM> connected to a lower part of the test pot <NUM>.

The gas permeability index (JPU) is a direct indicator of the quality of sintered ore, as it evaluates the gas permeability of a sintering raw material charged layer at the point when granulated matter (granulated raw material for sintering) is charged onto a pallet of a sintering machine. Measuring the particle size distribution of the granulated matter that is one of the strongest factors affecting the JPU is effective for improving the JPU.

Next, as the indicator of the particle size distribution, the ratio of coarse particles as in Patent Literature <NUM> is not adequate, and an indicator that can evaluate also a decrease in the gas permeability due to voids being filled up by fine particles is desirable. As such an indicator of the particle size distribution, the present inventers have adopted Isp. Disclosed on pages <NPL>et al. , Isp can be obtained from Dp = <NUM> / Σ(wi / di), where wi is a particle ratio, di is a particle diameter, Is = Dp<NUM>Σwi (<NUM> / di - <NUM> / Dp)<NUM>, Ip = (<NUM> / Dp)<NUM>Σwi(di - Dp)<NUM>, and Isp = <NUM> × √(Is x Ip). Is represents dispersion in a range of small particle diameters relating to variation in the specific surface area of particles, and Ip represents dispersion in a range of large particle diameters relating to variation in the particle diameter. As Isp is a geometric mean of Is and Ip and reflects both the dispersion in the range of small particle diameters and the dispersion in the range of large particle diameters, it can be said to be desirable as an indicator of gas permeability. That the value of Isp is closer to zero means that the particle size distribution is concentrated in a narrower range, which is an indicator of good gas permeability with fewer gaps being filled up by particles with different particle diameters.

In addition to considering the droplet diameter of water for the moisture to be added (water to be added) described above, considering what water to use is also an important perspective in the method of the present invention. Specifically, this was considered with a focus on whether to use, as the water to be used during granulation, simple water or so-called granulation water that is unique to the present invention and obtained by dispersing (including) fine powder in water beforehand.

It is generally known that fine powdery ore <NUM> composed mostly of particles with a particle diameter of not more than <NUM> like pellet feed turn into coarse particles (aggregated particles) with a low bonding force as small-diameter particles (fine powder) thereof aggregate during granulation. As schematically shown in <FIG>, this is known to be caused by a phenomenon that an adhesion force σ due to surface tension γ of water expressed by the following Formula (<NUM>) becomes higher as the particle diameter becomes smaller. [Formula <NUM>] <MAT>.

Next, the present inventers have thought that, if an appropriate amount of particles with an even smaller particle diameter is used for granulation, a strong adhesion force σ may be produced and the particles may be expected to act as a binder <NUM> (binding agent). Here, to produce the adhesion force of the binder <NUM>, ultrafine powder with a particle diameter of not more than <NUM> is conceivable. However, particles with such a particle diameter are ultrafine powder close to floating particulate substance defined as having a particle diameter of not more than <NUM>, and are extremely slow to fall under their own weight in the air. Therefore, when these particles are charged into a granulator in a dry state, they add to the amount of particles that float and scatter. On the other hand, when these particles are charged into a granulator in a moistened state to prevent floating and scattering, ultrafine powder with a particle diameter of not more than <NUM> does not disperse over the surfaces of other component materials of the granulated raw material for sintering, and thus the action of the binder <NUM> is lessened.

Therefore, the present inventers have decided to use, as the water to be added during granulation, granulation water obtained by adding ultrafine powder with a particle diameter of not more than <NUM> beforehand, and set the droplet diameter of this granulation water to a preferable range. We have newly learned that using such granulation water can disperse the ultrafine powder with a particle diameter of not more than <NUM> over the surfaces of other component materials of the granulated raw material for sintering, and at the same time can improve the particle size distribution after granulation and increase the gas permeability.

As a raw material containing a large amount of ultrafine powder with a particle diameter of not more than <NUM>, for example, iron-making dust of which a harmonic mean of primary particle diameters is about <NUM> to <NUM>, or a material obtained by further pulverizing concentrate powder or pellet feed obtained by ore dressing of iron ore can be used.

Methods to be used in granulating compounded raw material for sintering containing a large amount of ultrafine powder with a particle diameter of not more than <NUM> include a method of directly charging the ultrafine powder into the granulator along with other component materials of the granulated raw material for sintering and a method of mixing the ultrafine powder into water before being added. For example, in the granulation method of charging the ultrafine powder into the granulator along with other compounded raw material for sintering, while ultrafine powder with a particle diameter of <NUM> or smaller tends to float and scatter as described above, charging the ultrafine powder into the granulator at the same time as other component materials of the granulated raw material for sintering having large particle diameters can mitigate the floating and scattering. Weakly aggregating the ultrafine powder by slightly moistening it beforehand can also mitigate the floating and scattering.

On the other hand, the method of adding the ultrafine powder in a state of the granulation water obtained by mixing it into water beforehand has an advantage in that, not only are scattering and floating considerably mitigated, but also the use yield of ultrafine powder with a particle diameter of not more than <NUM> is good. Another advantage is that, since the ultrafine powder can be added so as to disperse over the granulation raw material inside the granulator, generation of coarse particles composed of fine particles and generation of fine particles due to insufficient granulation can be mitigated to narrow the particle size distribution of the particles having been granulated.

As another method of the present invention that uses granulation water containing ultrafine powder, quasi-ultrafine powder with a particle diameter exceeding <NUM> may be used by being mixed into ultrafine powder with a particle diameter of not more than <NUM>. However, when the number of particles with a large diameter increases, it becomes difficult to finely adjust the droplet diameter of the granulation water, and when the amount of particles in the granulation water is too large, it becomes difficult to transport the granulation water. It is therefore desirable that the amount of the ultrafine powder with a particle diameter of not more than <NUM> is more than <NUM> mass% relative to the total weight of particles to be mixed into the granulation water. There is an appropriate range for the amount of water used for granulation (normally <NUM> to <NUM> mass% relative to granulation raw material). Therefore, the ratio of the weight of the ultrafine powder with a particle diameter of not more than <NUM> relative to the total weight of the granulation water and the particles to be mixed into the granulation water is desirably at least <NUM> mass%, and the higher the ratio, the better.

In this method, the ratio of the ultrafine powder (particle diameter: not more than <NUM>) used by being mixed into the granulation water to be sprayed into the granulator relative to the total amount of sintering raw material charged into the granulator is not less than <NUM> mass% and not more than <NUM> mass%. When this ratio is not less than <NUM> mass%, the effect of narrowing the particle size distribution of particles having been granulated can be stably produced. On the other hand, when this ratio exceeds <NUM> mass%, a mixed liquid (granulation water) of the water and the ultrafine powder noticeably exhibits a non-Newtonian flow, so that the droplet diameter varies greatly and the workings and effects of the present invention are impaired.

Next, the present inventers have studied about the position of adding water or granulation water in a length direction of the granulator. It is known that granulated particles having extremely large diameters after granulation have low strength and therefore are not desirable. It is also known that when the granulated particles vary widely in diameter after granulation, the gas permeability decreases as small-diameter particles fill up gaps between large-diameter particles, which impairs the productivity of the sintering machine. Therefore, in addition to preventing the granulated particles from having extremely large diameters after granulation, it is also important to reduce the variation, in other words, mitigate the generation of granulated particles that are extremely small.

To this end, it is desirable that the position of adding moisture inside a granulator like a drum mixer is at a stage preceding a state where granulated particles immediately before being discharged have grown sufficiently. In view of this, it is desirable that, with positions of a part for charging raw material before granulation (inlet) and a part for discharging granulated matter after granulation (outlet) of the granulator in the length direction being indicated as <NUM>% and <NUM>%, respectively, the moisture is supplied to the compounded raw material for sintering at a position within a range of <NUM>% to <NUM>%. Further, it is desirable that the position of adding the moisture is a position after the compounded raw material for sintering that was uneven immediately after charging have been evenly mixed. In view of these points, it is desirable that the moisture is supplied to the compounded raw material for sintering at a position within a range of <NUM>% to <NUM>% in the length direction of the granulator, and it is further desirable that the moisture is supplied at a position within a range of <NUM>% to <NUM>%.

When the water to be added into the granulator is granulation water containing ultrafine powder, it is desirable that the granulation water is added at a position within a range of <NUM> to <NUM>%, a little farther downward than in the case of water alone, and it is more desirable that the granulation water is added at a position within a range of <NUM> to <NUM>%. This is because the binder effect by the ultrafine powder contained in the granulation water is exhibited more noticeably when fine powder and ultrafine powder inside the compounded raw material for sintering charged into the granulator have aggregated to some extent.

In the present invention, it is important to evenly disperse water into the compounded raw material for sintering being granulated. Therefore, it is desirable that droplets of water is added so as to directly reach a sloping surface of the accumulated compounded raw material for sintering inside the granulator, instead of reaching an inner wall surface of the granulator. In particular, as shown in <FIG>, it is further desirable that a center C of a spraying direction of a spray nozzle that sprays the water (granulation water) is directed at the sloping surface of the accumulated compounded raw material for sintering located within a range R of not less than <NUM>° and not more than <NUM>° in a drum rotation direction, with <NUM>° being the vertically downward direction, in a cross-section of the drum mixer perpendicular to a rotational axis, i.e., the range R is a preferred range of a direction in which the center of spraying of the granulation water is directed at the granulation raw material. This is because, compared with when the water (granulation water) is added so as to be focused on a range of not less than <NUM>° and not more than <NUM>°, when the water is added so as to be focused on the range of not less than <NUM>° and not more than <NUM>°, the particles of the compounded raw material for sintering slide down a longer distance while rotating over the sloping surface of the accumulated compounded raw material for sintering after the droplets reach the surfaces of particles of the compounded raw material for sintering, and thus dispersion of water is promoted also after the droplets reach the compounded raw material for sintering.

Generally, the moisture content in compounded raw material for sintering being granulated inside a granulator affects the particle diameter and the strength of granulated material and is therefore managed to be within a predetermined range. However, in compounded raw material for sintering before being charged into the granulator, there are some brands of materials that contain moisture due to the influence of rain etc. in the course of storage in an outdoor yard, and that vary in moisture content due to the influence of the weather, the storage period, etc. This makes it necessary to adjust the amount of water added inside the granulator according to the moisture content of the compounded raw material for sintering before being charged into the granulator. Moreover, since it is important in the present invention to disperse water inside the compounded raw material for sintering in granulation, it is not desirable to reduce the amount of water added into the granulator even when some brands of the compounded raw material for sintering before being charged into the granulator have a high moisture content. Therefore, to secure a fixed amount of water to be added into the granulator, it is desirable to adjust the mean moisture content of the compounded raw material for sintering before being charged into the granulator to not more than <NUM> mass% beforehand, and it is further desirable to adjust the mean moisture content to not more than <NUM> mass%. Then, the effect of the present invention of adding water in the granulator is enhanced. As a method for adjusting the mean moisture content of the compounded raw material for sintering before being charged into the granulator, the raw materials may be appropriately mixed and adjusted to the aforementioned water content (not more than <NUM>. 5mass%) on the assumption that the moisture content varies according to the brand or the period of storage in an outdoor yard.

A granulation test by spraying water in a laboratory and measurement of the resulting droplet diameters were performed. As an experiment device, compounded raw material for sintering was charged into a drum mixer with a diameter of <NUM> and granulation was performed while water was sprayed. In this case, the droplet diameter was controlled by changing the type of nozzle. Quasi-particles after granulation were used for a gas permeability test and the degree of air permeation was measured. The droplet diameters were measured by the method of performing an image analysis of <NUM> droplets using a high-speed camera and calculating an arithmetic mean dimeter.

The result of this test is shown in <FIG>. Compared with a comparative example in which no nozzle was used and a large part of water was a liquid column, an improving effect on the gas permeability was recognized with the droplet diameter within a range of not less than <NUM> to not more than <NUM>. Further, a greater improvement was recognized with the droplet diameter within a range of not less than <NUM> to not more than <NUM>, and an even greater improvement was recognized with the droplet diameter within a range of not less than <NUM> to not more than <NUM>.

Under the same conditions as in Reference Example <NUM>, iron-making dust of which predominant particles had a particle diameter of not more than <NUM> was mixed into water at a ratio of <NUM> to <NUM> of water to be added to obtain granulation water. This granulation water was sprayed at a droplet diameter of <NUM> and the particle diameter after granulation was measured. The particles with a particle diameter of not more than <NUM> that were sprayed into the granulator by being mixed into the granulation water were set to <NUM> mass% of the total amount of sintering raw material charged into the granulator.

The result is shown in <FIG>. Example <NUM> in <FIG> shows values obtained by spraying only water without iron-making dust mixed into it at a droplet diameter of <NUM>. The comparative example in <FIG> shows values obtained by adding only water without iron-making dust mixed into it in a state where major part of the water was a liquid column. As is clear from <FIG>, the spread of the particle size distribution is: the comparative example > Reference Example <NUM> > Example <NUM>. While Isp that is an indicator of the spread of the particle size distribution is <NUM> in the comparative example, it is improved to <NUM> in Reference Example <NUM> and significantly improved to <NUM> in Example <NUM>.

When the ratio of particles having a particle diameter of not more than <NUM> that were sprayed into the granulator by being mixed into the granulation water relative to the total amount of sintering raw material charged into the granulator was not less than <NUM> mass% and not more than <NUM> mass%, Isp lower than that of Reference Example <NUM> was obtained.

Claim 1:
A method for producing sintered ore that obtains sintered ore by granulating compounded raw material for sintering including iron ore powder of multiple types of brands by a granulator and sintering obtained granulated raw material for sintering by a sintering machine, wherein
not less than <NUM> mass% of moisture to be added during granulation is supplied at a mean droplet diameter of not less than <NUM> and not more than <NUM>,
characterized in that
the moisture contains fine particles with a particle diameter of not more than <NUM> at a ratio of not less than <NUM> mass% and not more than <NUM> mass% relative to the compounded raw material for sintering, and
wherein the mean droplet diameter is measured by performing an image analysis of not less than <NUM> droplets using a high-speed camera and calculating the arithmetic mean diameter.