TWI858647B - Method for producing granular iron ore - Google Patents

Abstract

Provides technology that can efficiently produce iron ore pellets with lower P concentration.

The method comprises: a first step of melting reduced iron to form a primary molten iron, a second step of separating the primary molten iron from slag, a third step of dephosphorizing the primary molten iron separated from the slag to form a secondary molten iron, and a fourth step of solidifying the secondary molten iron into granules and mineralizing the granular iron; in the third step, an oxygen source and a CaO source are supplied to the primary molten iron to perform a dephosphorization treatment, and the temperature of the secondary molten iron at the end of the dephosphorization treatment is formed to be lower than the temperature of the primary molten iron at the start of the dephosphorization treatment.

Description

Method for producing granular iron ore

The present invention relates to granular iron ore with reduced P concentration and a method for producing the same.

In recent years, the need to expand the use of cold iron sources (metal scrap) in the iron smelting industry has increased. In order to build a sustainable society, the recycling of iron sources is necessary and inevitable. In addition, from the perspective of preventing global warming, even from the perspective of reducing _CO2 emissions, the increase in the use of metal scraps is indispensable. Unlike iron ore in the form of iron oxide ( _{Fe2O3 )} , metal scraps do not require a reduction step in the smelting process, which can reduce _CO2 emissions. Therefore, the use of cold iron sources is constantly increasing.

The blast furnace-converter process is a steelmaking process in which raw material iron ore (Fe ₂ O ₃ ) and a reducing agent, namely coal coke (carbon source), are charged into a blast furnace to melt molten iron with a carbon concentration of about 4.5-5%, and the molten iron is charged into a converter to oxidize and remove impurities, namely C, Si, and P. When molten iron is produced in a blast furnace, about 500 kg of carbon source is required for each ton of molten iron in order to reduce the iron ore. In addition, about 2 tons of CO ₂ gas is generated. On the other hand, when molten steel is produced using iron metal scrap as a raw material, the carbon source required for the reduction of iron ore is not required. Therefore, even if the energy required to melt ferrous scrap is taken into account, replacing 1 ton of molten iron with 1 ton of ferrous scrap will reduce _CO2 emissions by about 1.5 tons. Due to the above situation, in order to balance the reduction of greenhouse gas emissions and maintain production activities, the use of ferrous scrap must be increased.

However, due to the tight supply and demand of ferrous scrap, especially high-grade ferrous scrap which is indispensable for high-grade steel production, there is an increasing demand for reduced iron as a substitute for metal scrap. Reduced iron is produced by reducing iron ore, but it does not need to increase the carbon concentration in the produced iron as in the blast furnace-converter process, and does not use excessive carbon sources, which reduces _CO2 emissions by about 0.2 tons per ton of iron. In addition, _CO2 emissions can be further reduced by replacing the carbon source with a hydrocarbon gas such as hydrogen or natural gas as the reducing agent.

As an issue when using reduced iron, for example, reduced iron contains phosphorus. Since phosphorus in iron and steel products causes a decrease in quality such as hot brittleness, the phosphorus concentration must be reduced to a level corresponding to the required quality. However, when reducing iron is melted in an electric furnace to produce molten iron, most of the phosphorus in the reduced iron enters the molten iron (also called complex phosphorus). Therefore, the current reduced iron is produced from high-grade iron ore (P concentration of about 0.01 mass%) with a low P concentration in iron ore, and the P concentration of reduced iron is about 0.02 mass%.

On the other hand, the depletion of high-grade iron ore with low P concentration is expected, and in the future, it is required to produce molten steel using reduced iron from low-grade iron ore with high P concentration as raw material. The P concentration in iron ore used in the current blast furnace method is 0.05~0.10 mass% (converted to 0.10~0.15 mass% P concentration as reduced iron), and it is expected that the P concentration will increase further in the future. This phosphorus concentration is 5 to 10 times higher than the phosphorus concentration of reduced iron produced from the aforementioned high-grade iron ore with a lower phosphorus concentration. In order to prevent the quality of iron and steel products from being degraded due to phosphorus in them, phosphorus must be removed when reducing iron with a higher phosphorus concentration is melted to produce molten steel, or when reducing iron is produced from iron ore with a higher phosphorus concentration. Several technologies have been proposed for such phosphorus removal technologies.

Patent document 1 proposes a flux for dephosphorization refining in an electric arc furnace, which is mainly composed of calcium oxide, 5 to 15 mass % of aluminum oxide, 25 to 35 mass % of iron oxide, and the remainder being inevitable impurities, for removing phosphorus from molten steel to a low concentration in a relatively short time using an electric arc furnace alone.

In addition, Patent Document 2 proposes a method for removing phosphorus by bringing an iron ore, a titanium-containing ore, a nickel-containing mine, a chromium-containing mine, or a mixture containing these mines as main components, having a CaO content of 25 mass % or less and a CaO/(SiO ₂ +Al ₂ O ₃ ) ratio of 5 or less into contact with one of Ar, He, N ₂ , CO, H ₂ , and hydrocarbons, or a mixed gas thereof, at a temperature of 1600° C. or above.

Patent document 3 proposes a method for separating and recovering calcium phosphate by crushing an iron ore with a high P concentration to less than 0.5 mm, adding water to the ore to make a slurry with a concentration of about 35 mass %, adding H ₂ SO ₄ or HCl to the solvent and reacting it at a pH of less than 2.0, then collecting magnetically adsorbed materials such as magnetic iron ore by magnetic classification, and separating non-magnetic adsorbed materials such as SiO ₂ and Al ₂ O ₃ as slurry, and adding slaked lime or quicklime to the P dissolved in the liquid and neutralizing it within the pH range of 5.0 to 10.0.

[Prior Art Literature] [Patent Literature]

Patent document 1: Japanese Patent Publication No. 8-120322

Patent document 2: Japanese Patent Publication No. 54-83603

Patent document 3: Japanese Patent Publication No. 60-261501

However, the above-mentioned existing technologies have the following problems that must be solved.

In other words, in the technology disclosed in Patent Document 1, an iron source with a low P concentration such as metal scrap is assumed. Specifically, in order to reduce the P concentration in the molten steel from 0.020 mass% to 0.005 mass%, 350g of flux is added to 7000g of molten steel. If the P concentration in the reduced iron is assumed to be 0.15 mass%, the amount of flux required to reduce the P concentration to 0.01 mass% at the level of iron and steel products is 230kg per 1t of molten steel. In this way, there is a problem that the volume ratio of the flux in the arc furnace increases, the amount of molten steel processed decreases, and the manufacturing efficiency deteriorates.

The method disclosed in Patent Document 2 has a processing temperature of 1600°C or above, and in the specification, it is stated that "in order to perform dephosphorization more effectively, it is preferably above 1800°C. Such a high temperature range is difficult to achieve by conventional heating methods, but can be achieved by using, for example, plasma arc and high-frequency induction plasma." Therefore, a large amount of energy is required and it is not suitable for such topics as complicated processing.

The method disclosed in Patent Document 3 is a wet treatment using acid, which is useful for the problem that the drying of the recovered magnetic adsorbent material used as the main raw material requires time and cost. In addition, this method still has the problem that it needs to be crushed to less than 0.5 mm in advance, which also requires time and cost.

The present invention is made in view of the above-mentioned problems, and its purpose is to provide a technology that can efficiently produce iron ore particles with a lower P concentration even if the raw material is reduced iron obtained from low-grade iron ore with a higher P concentration.

The method for producing granular iron ore according to the present invention, which is useful for solving the above-mentioned problems, is characterized by comprising: a first step of melting reduced iron to form a primary molten iron, a second step of separating the primary molten iron from slag, a third step of dephosphorizing the primary molten iron separated from the slag to form a secondary molten iron, and a fourth step of solidifying the secondary molten iron into granular iron ore, wherein in the third step, an oxygen source and a CaO source are supplied to the primary molten iron to perform dephosphorization, and the temperature of the secondary molten iron at the end of the dephosphorization is formed to be lower than the temperature of the primary molten iron at the start of the dephosphorization.

In addition, the method for producing iron ore pellets according to the present invention is considered to have the following better solutions: (a) the temperature _Tf of the secondary molten iron at the time of the completion of the dephosphorization treatment is increased by 20°C or more from the solidification temperature _Tm of the secondary molten iron at the time of the completion of the dephosphorization treatment; (b) the composition of the slag at the time of the completion of the dephosphorization treatment is increased relative to the _SiO2 concentration (% _SiO2 ) is set within the range of 1.0 to 4.0; (c) in the aforementioned fourth step, a granular iron ore manufacturing device is used, and the granular iron ore manufacturing device comprises: a granulating device, which converts the aforementioned secondary molten iron into droplets; a water flow control container, which is arranged at a position to receive the aforementioned droplets and contains cooling water; and at least one cooling water pipe, which is connected to the aforementioned water flow control container and supplies cooling water to the aforementioned water flow control container, the aforementioned water flow control container has an inclined surface which is inclined downward and in a manner that the horizontal cross-sectional area of the aforementioned water flow control container becomes narrower, and a discharge outlet is arranged below the aforementioned inclined surface.

The granular iron ore involved in the present invention, which is beneficial to solving the above-mentioned problems, is characterized by: reduced iron with a P concentration of more than 0.050 mass % as raw material, a P concentration of less than 0.030 mass %, and a particle size of more than 1 mm and less than 50 mm.

According to the method for producing granular iron ore of the present invention, granular iron ore with a lower P concentration can be efficiently produced from reduced iron obtained from low-grade iron ore with a higher P concentration. In addition, the granular iron ore of the present invention satisfies the P concentration required by many iron and steel products, in other words, less than 0.030 mass%. Therefore, molten iron with a P concentration equivalent to that of iron and steel products can be obtained by remelting the granular iron ore of the present invention.

Furthermore, since the dephosphorized ore solidifies into granules in a short time as granular iron ore, it is possible to produce steel in a form where the iron source production plant and the demand site are separated without using a large-scale iron smelter. For example, it is possible to consider a form in which dephosphorized granular iron ore is produced in the raw material producing country, and steel is produced in the steel producing country using the dephosphorized granular iron ore as raw material.

In addition, when the production of reduced iron and the production of iron ore pellets involved in the present invention are carried out in the iron ore producing country, the amount of vein stone contained in the raw iron ore can be separated as slag. Therefore, by transporting only iron ore pellets, the transportation volume per unit Fe amount can be reduced, and the transportation cost and energy consumption to the demand site can be suppressed. In addition, as long as the particle size of the iron ore pellets is 1~50mm, when considering the transportation and storage of the iron ore pellets to the demand site, the degree of freedom of the equipment used for transportation, storage and supply is increased when supplying to the equipment at the demand site. In addition, there is also an effect of suppressing the risk of hanging in the supply hopper.

The following is a detailed description of the implementation of the present invention. In addition, the following implementation is used to list examples of devices and methods that embody the technical ideas of the present invention, and does not constitute a specific example. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the patent application scope.

The inventors made the following considerations when making this invention.

Reduced iron produced from iron ore as raw material has different characteristics such as metallization rate and composition depending on the brand of the iron ore used, the type and basic unit of the raw material composition modifier mixed, the type and basic unit of the reducing agent, the reduction temperature, and the type of reduced iron manufacturing equipment. Table 1 shows an example of the composition of reduced iron. In the example of Table 1, the P concentration converted to molten iron by dividing the P concentration by the T.Fe (total iron) concentration is 0.057~0.152 mass%. Therefore, even if these reduced irons are directly melted, it is still difficult to achieve a P concentration of steel product grade (less than 0.030 mass%). In addition, when simply melting reduced iron for dephosphorization, there are problems such as a large amount of slag generated due to the ore such as _SiO2 contained in the reduced iron, a large proportion of the slag relative to the volume of the treatment equipment, a decrease in productivity, and an increase in the amount of CaO source required to ensure the amount of phosphorus removed from the molten iron, which increases the treatment cost.

Therefore, a process for producing granular iron ore was designed, in which reduced iron is melted in a short time to obtain molten iron, at least a portion of the slag caused by the ore is removed, an oxygen source and a lime source are supplied to the obtained molten iron to dephosphorize it, and then the molten iron is solidified into granules after dephosphorization.

The following is a detailed description of the implementation of the present invention.

As a first step, the temperature of reduced iron is increased and melted in an electric furnace to obtain primary molten iron. Reduced iron, for example, can be produced in an adjacent reduced iron production plant and directly transported at high temperature for short distances. Of course, products that have been cooled to room temperature in a short time can also be used. The electric furnace can also be any of an arc furnace, a submerged arc furnace, or an induction melting furnace. In addition, the thermal energy used to melt the reduced iron that is a solid iron source in the first step and to heat the iron source is not only electrical energy, but also gas fuels such as natural gas and propane gas, liquid fuels such as heavy oil, coal, and the combustion heat of combustible solids such as metal Al and Si can be used supplementarily. The fact that these energy sources are renewable is better from the perspective of reducing CO ₂ emissions.

As a second step, the slag containing the vein of the reduced iron is separated from the primary molten iron. For example, it flows out of a container for transporting molten metal and is transported to a facility for dephosphorization. In the subsequent steps, since a CaO source is added to generate slag for dephosphorization, at least a portion of the large amount of slag containing _SiO2 generated by the melting of the reduced iron may be retained in order to ensure the slag amount and adjust the composition. The slag may also be removed from the melting and heating container used in the first step using a slag skimmer or the like.

As the third step, the molten metal is dephosphorized to obtain secondary molten iron. The dephosphorization reaction requires an oxygen source and a CaO source as shown in the following formula (1).

2[P]+5/2. O ₂ (g)+3CaO(s)=3CaO． P ₂ O ₅ (s). ． ． (1)

Here, [P] represents phosphorus in molten iron.

Pure oxygen gas is generally used as the oxygen source for dephosphorization. Considering that dephosphorization is an exothermic reaction, it is more advantageous to perform dephosphorization at a low temperature. In addition, in the next step, it is more advantageous to lower the molten iron temperature within a range where there is no problem in the treatment in order to solidify the molten iron and mineralize the granular iron.

As a result of the investigation, it was found that by supplying air or an iron oxide source such as iron ore and iron oxide scale as an oxygen source, the molten iron can be dephosphorized sufficiently while cooling. Regarding the use of air, the sensible heat of nitrogen gas contained in the air can be discharged to obtain a cooling effect for the case of using pure oxygen gas. In addition, regarding the use of the iron oxide source, the iron oxide source is reduced to metal Fe in an endothermic reaction, or the molten slag is formed in the form of iron oxide, and the cooling effect for the case of using pure oxygen gas is obtained.

Next, since calcium carbonate contained in limestone absorbs heat when it decomposes into CaO and _CO2 , the molten iron can be cooled by using limestone as a CaO source. Although the same cooling effect can be obtained by supplying carbonates such as raw dolomite, when the CaO ratio in the auxiliary raw material becomes lower, the amount of auxiliary raw materials added increases, the amount of slag generated increases, or the time required for addition becomes longer, which causes operational problems. Therefore, it is better to adjust the type and amount of auxiliary raw materials added in consideration of the required cooling effect and stable operation.

Since the spraying pattern varies depending on the water outlet height of the dephosphorization vessel (the height from the molten iron surface to the upper end of the vessel) and the nozzle shape of the top-blowing lance, it is preferable to adjust the supply rate of pure oxygen or air and the lance height according to the operation status of the dephosphorization process. In addition, it is preferable to blow in an inert gas in order to agitate the molten iron. The inert gas is preferably blown in through a porous plug installed at the bottom of the furnace or by dipping the injection lance into the molten iron. The composition of the slag at the end of the dephosphorization treatment is preferably such that the ratio of the CaO concentration (% _CaO ) to the _SiO2 concentration (%SiO2) is in the range of 1.0 to 4.0 in terms of slag alkalinity on a mass basis. The composition is adjusted by the amount of _SiO2 contained in the slag in large amounts in the second step and the type and amount of the added CaO source. _SiO2 sources such as silica and ferrosilicon, or CaO sources such as quicklime, may also be added as needed.

In addition, if the slag alkalinity is low, the amount of phosphorus removed during dephosphorization treatment becomes smaller. If the slag alkalinity is high, part of the slag solidifies and adheres to the refractory when the molten iron temperature drops, so it is difficult to remove the slag after dephosphorization treatment, and there may be problems such as abnormal reaction when charging molten iron in the next treatment, residual slag mixing with the generated slag and causing composition discrepancy.

In addition, since a large amount of high-temperature exhaust gas is generated during dephosphorization using such air, a boiler or the like can be used to recover the exhaust heat.

As the fourth step, the secondary molten iron after dephosphorization is solidified into granules to obtain granular iron ore. As a method for producing granular iron ore, for example, the molten iron after dephosphorization is made to flow down and collide with the table of refractory materials, or water is made to collide with the flowing molten iron to form granular molten iron, and the molten iron drops into a water flow control container to obtain solidified granular iron ore. Since the diameter of the granular iron ore changes according to the flow speed of the molten iron at this time, the molten iron after dephosphorization is preferably transferred to the middle trough where the flow speed can be kept constant.

At this time, the temperature of the molten iron drops during the transportation of the dephosphorized molten iron and the supply to the iron ore pelletizing device. When the temperature of the molten iron after the dephosphorization treatment is too low, part of the molten iron solidifies before all the molten iron in the container is supplied to the iron ore pelletizing device, and the manufacturing yield decreases. On the other hand, if the temperature of the molten iron after the dephosphorization treatment is high, the heat load during solidification in the iron ore pelletizing device often increases, and the amount of cooling water used increases, etc., and the cooling speed becomes an obstacle and the productivity decreases, or the temperature of the molten iron drops, resulting in a longer standby time before the iron ore pelletizing device. As described above, when the ore-forming of the iron nuggets after dephosphorization is assumed, there is a range of the molten iron temperature suitable for the iron nuggets after dephosphorization. Specifically, from the viewpoint of productivity, the molten iron temperature _Tf after dephosphorization is set to be lower than the molten iron temperature _Ti at the start time of the dephosphorization. In addition, when the temperature _Tf at the end time of the dephosphorization is increased by 20°C or more from the solidification temperature _Tm of the secondary molten iron at the end time of the dephosphorization, it is preferable because the molten iron can be supplied to the iron nuggets manufacturing device at a high productivity.

Here, the solidification temperature _Tm (°C) may be a method of directly measuring the solidification temperature of a sample, or a method of obtaining the temperature from the liquidus temperature of the Fe-C phase diagram from the C concentration of the molten iron after dephosphorization estimated from past operating results (C concentration before dephosphorization, temperature, type and supply conditions of oxygen source, etc.).

The granular iron ore manufacturing device is provided with: a granulating device for turning molten iron into liquid droplets; and a water flow control container which is arranged at a position to receive the liquid droplets and contains cooling water. At least one cooling water pipe is connected to the water flow control container which supplies cooling water to the molten iron to make it fall and solidify. By spouting cooling water from the cooling water pipe, a water flow is generated to suppress the formation of a precipitation area of cooling water in the container. Therefore, the local temperature rise of the cooling water is suppressed and the granular iron ore can be efficiently cooled, thereby suppressing the situation where the granular iron ore is not sufficiently cooled and melts with each other. In addition, the water flow control container has an inclined surface which is inclined downward and in a manner that the horizontal cross-sectional area of the container becomes narrower, and the bottom of the inclined surface serves as a discharge port. By making the inclination angle of the inclined surface larger than the repose angle of the iron ore in the water, the iron ore can be guided to the discharge port without being retained on the inclined surface.

When the iron ore pellets obtained as described above are used as part of the iron source in a blast furnace or converter, the P concentration can be diluted according to the proportion used. This can reduce the load in the dephosphorization treatment and ease the constraints on the raw materials used in the blast furnace and converter.

Furthermore, when the iron ore pellets obtained by the present embodiment are used as an iron source in an electric furnace, a blast furnace, and a converter, there is a range of particle sizes that are easy to use. Therefore, it is better to adjust the flow rate in the middle chute to obtain the desired particle size. In addition, it is better to grade it as needed. Generally speaking, it is easier to use a particle size in the range of 1 to 50 mm. In the case of iron ore pellets with a particle size less than 1 mm, the possibility of clogging the conveyor used for transportation and hanging in the hopper increases, so it is better to use one that is graded to 1 mm or more. In addition, if the particle size exceeds 50 mm, when a collision caused by falling occurs, the risk of damaging equipment such as the conveyor and hopper used for transportation increases. Therefore, it is better to make the particle size less than 50mm and reduce the flow rate in the middle trough. In addition, it is also possible to classify and remove iron ore particles exceeding 50mm as needed. Here, the range of particle size 1~50mm refers to the upper screen of the screen with a mesh size of 1mm and the lower screen of the screen with a mesh size of 50mm.

[Implementation example] (Implementation Example 1)

The reduced iron A shown in Table 1 is melted in a 250t electric furnace and transferred to a pot-shaped container after adjusting the temperature. At this time, about 10kg/t-molten iron of the slag generated when the reduced iron is melted in the electric furnace due to the vein stone contained in the reduced iron is transferred to the pot-shaped container together, and the rest is transferred to another slag container. The pot-shaped container is moved to the dephosphorization treatment equipment, and the type and amount of the supplied oxygen source and lime source are changed to perform dephosphorization. The dephosphorization treatment equipment has: a gas top blowing nozzle, an auxiliary raw material transfer hopper, and a bottom blowing porous plug. A gas containing pure oxygen or air can be supplied from the gas top blowing nozzle at a rate of about 1Nm ³ /(min‧t-molten iron). There are three auxiliary material transfer hoppers, which are filled with iron ore, quicklime (CaO), and calcium carbonate (CaCO ₃ ), respectively, and can be supplied at a rate of about 10 kg/min. Gas can be supplied from the bottom-blown porous plug, and in this embodiment, pure Ar gas is supplied at a rate of about 0.1 Nm ³ /(min‧t-molten iron).

The melting temperature in the electric furnace is adjusted so that the temperature of the molten iron before dephosphorization is about 1590℃. The timing before the gas top-blowing gun is lowered and the timing when the gas top-blowing gun is raised after the treatment are respectively regarded as before and after the dephosphorization treatment, and the temperature measurement and sampling are respectively carried out using the auxiliary gun. The sampled samples are cut and ground, and the C concentration [C] and P concentration [P] in the molten iron are evaluated from the pre-made calibration curve by the emission spectrometry method. In addition, the solidification temperature of the molten metal during the timing of the auxiliary gun temperature measurement and sampling can be measured, and the solidification temperature _Tm of the molten iron after the dephosphorization treatment can be measured.

The dephosphorization treatment starts when the gas top-blowing gun descends. When the top-blowing gun reaches a predetermined height, the supply of oxygen gas source and the addition of auxiliary materials start. After the supply of predetermined amounts of oxygen gas source and auxiliary materials is completed, the dephosphorization treatment ends when the top-blowing gun rises to the standby position. This period is the treatment time _tf (minutes).

After dephosphorization, the pot-shaped container is tilted and the slag on the molten iron is removed by a slag scraper. A portion of the removed slag is collected for chemical analysis. Afterwards, the pot is lifted and tilted by a crane, and the molten iron is transferred to the middle trough. The molten iron flows down from the middle trough and hits the refractory table to form droplets, which fall into the water flow control container and solidify to produce granular iron ore. The particle size of the obtained granular iron ore is 0.1~30mm. The particle size distribution is +0.1mm-1mm: 17.2 mass%, +1mm-10mm: 31.3 mass%, +10mm-20mm: 38.8 mass%, +20mm-30mm: 12.7 mass%. Here, +N-M means that the mesh size N is the upper part of the screen, and the mesh size M is the lower part of the screen.

In Table 2, the molten iron temperatures _Ti and _Tf (°C) before and after dephosphorization treatment, C concentrations [C] _i and [C] _f (mass %), P concentrations [P] _i and [P] _f (mass %), the types and amounts of oxygen source and CaO source supplied, treatment time _tf (min), and the alkalinity of the slag after treatment (the ratio of the mass-based CaO concentration (%CaO) to the _SiO2 concentration (% _SiO2 ) (%CaO)/(% _SiO2 ), hereinafter referred to as C/S) are shown for treatments No. 1 to 5.

As shown in Table 2, in any of the inventive examples, the molten iron temperature _Tf after treatment is lower than the molten iron temperature _Ti before treatment, and the P concentration [P] _f after treatment is sufficiently reduced. The comparative example is the result that the temperature _Tf after dephosphorization treatment is higher than the temperature _Ti before dephosphorization treatment, and the P concentration [P] _f after treatment is increased, and a waiting time is generated in the step of producing granular iron ore, and productivity is reduced. In addition, compared with treatment No. 1 to 3, in treatment No. 4, since the molten iron temperature _Tf after treatment is reduced, although the P concentration [P] _f is sufficiently reduced, a part of it solidifies in the middle drain tank during the production of granular iron ore, and the productivity is reduced. In treatment No. 1 to 3, the molten iron temperature _Tf after treatment is lower than the molten iron temperature _Ti before treatment, and the molten iron temperature _Tf after treatment is increased by more than 20°C from the solidification temperature _Tm of the molten iron. The P concentration [P] _f after treatment is sufficiently reduced, and the yield is good and the full amount of granular iron ore can be produced without reducing productivity.

(Example 2)

Dephosphorization and pelletized iron ore production were performed in the same manner as in Example 1. Table 3 shows the molten iron temperatures _Ti and _Tf (°C) before and after dephosphorization, C concentrations [C] _i and [C] _f (mass %), P concentrations [P] _i and [P] _f (mass %), the types and amounts of oxygen source and CaO source supplied, treatment time _tf (minutes), and alkalinity C/S of the slag after treatment for treatment No. 6 to 12. As shown in Table 3, the P concentration after treatment was high in treatment No. 11 because the alkalinity C/S of the slag was low compared to treatment No. 6 to 10. In addition, in treatment No. 12, the alkalinity C/S of the slag was high and solidification of the slag was confirmed.

(Implementation Example 3)

In a 250 t electric furnace, the reduced iron A shown in Table 1 was melted together with anthracite to form a molten iron containing a C concentration of about 2.0 mass %, and then transferred to a pot-shaped container with the temperature adjusted. Thereafter, dephosphorization treatment and pelletized iron ore production were performed using the same method as in Examples 1 and 2. In Table 4, the molten iron temperatures _Ti and _Tf (°C) before and after dephosphorization treatment, C concentrations [C] _i and [C] _f (mass %), P concentrations [P] _i and [P] _f (mass %), the types and amounts of oxygen sources and CaO sources supplied, the treatment time _tf (minutes), and the alkalinity C/S of the slag after treatment are shown in Treatment Nos. 13 to 19. As shown in Table 4, the alkalinity C/S of the slag in treatment No. 18 is lower than that in treatment No. 13 to 17, so the P concentration [P] _f after treatment is high. In addition, in treatment No. 19, the alkalinity C/S of the slag is high and solidification of the slag is confirmed.

The iron ore nuggets produced by treatment No. 8~10, 12, 14~17 and 19 have a P concentration of 0.030 mass% or less. When these reduced irons are melted in an electric furnace, the P concentration of the resulting molten iron becomes 0.030 mass% or less. These achieve the P concentration required for iron and steel products, and no additional dephosphorization treatment is required. In addition, after the iron ore nuggets obtained by treatment No. 8~10, 12, 14~17 and 19 are graded to 1mm or more, they can be used in electric furnaces, blast furnaces, and converters without any problems.

In this specification, the unit of mass "t" represents 10 ³ kg. In addition, N added to the unit of volume "Nm ³ " represents the standard state of gas, which is assumed to be 1 atm (=101325 Pa) and 0°C in this specification. [M] in the notation of a chemical formula represents that the element M is dissolved in molten iron and reduced iron.

[Possibility of industrial application]

According to the method for producing iron ore nuggets and the iron ore nuggets of the present invention, even if the raw material is reduced iron obtained from low-grade iron ore with a high P concentration, iron ore nuggets with a low P concentration can still be efficiently produced. Molten iron with a P concentration equivalent to that of iron and steel products can be obtained by remelting the iron ore nuggets involved in the present invention, so it is practical in industry.

Claims (4)

A method for producing granular iron ore, comprising: a first step of melting reduced iron to form a primary molten iron, a second step of separating the primary molten iron from slag, a third step of dephosphorizing the primary molten iron separated from the slag to form a secondary molten iron, and a fourth step of solidifying the secondary molten iron into granular iron ore, wherein in the third step, an oxygen source and a CaO source are supplied to the primary molten iron to perform dephosphorization, and the temperature of the secondary molten iron at the end of the dephosphorization is set to be lower than the temperature of the primary molten iron at the start of the dephosphorization. The method for producing iron ore nuggets as recited in claim 1, wherein the temperature _Tf of the secondary molten iron at the time point of completion of the dephosphorization treatment is increased by 20°C or more from the solidification temperature _Tm of the secondary molten iron at the time point of completion of the dephosphorization treatment. The method for producing iron ore pellets as recited in claim 1 or 2, wherein the alkalinity of the slag composition at the end of the dephosphorization treatment is set within a range of 1.0 to 4.0 in terms of the ratio of CaO concentration (%CaO) to _SiO2 concentration (% _SiO2 ) on a mass basis. The method for producing granular iron ore as described in claim 1, wherein in the aforementioned fourth step, a granular iron ore production device is used, and the granular iron ore production device has: a granulating device, which converts the aforementioned secondary molten iron into liquid droplets; a water flow control container, which is arranged at a position to receive the aforementioned liquid droplets and contains cooling water; and at least one cooling water pipe, which is connected to the aforementioned water flow control container and supplies cooling water to the aforementioned water flow control container, wherein the aforementioned water flow control container has an inclined surface that is inclined downward and in a manner that the horizontal cross-sectional area of the aforementioned water flow control container becomes narrower, and a discharge port is arranged below the aforementioned inclined surface.