Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B13/00—Making spongy iron or liquid steel, by direct processes
  • C21B13/0006—Making spongy iron or liquid steel, by direct processes obtaining iron or steel in a molten state
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B11/00—Making pig-iron other than in blast furnaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B13/00—Making spongy iron or liquid steel, by direct processes
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C1/00—Refining of pig-iron; Cast iron
  • C21C1/02—Dephosphorising or desulfurising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/064—Dephosphorising; Desulfurising
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
  • C21C7/04—Removing impurities by adding a treating agent
  • C21C7/064—Dephosphorising; Desulfurising
  • C21C7/0645—Agents used for dephosphorising or desulfurising

Definitions

• the present invention relates to grained iron with a reduced P concentration, and a method for producing the same.
• the blast furnace-converter method is a steelmaking process that includes supplying iron ore (Fe 2 O 3 ) as a raw material into a blast furnace together with coke (a carbon source) as a reducing agent, to obtain molten pig iron with a C concentration of approximately 4.5 to 5%, and then supplying the obtained molten pig iron into a converter to remove C, Si, and P, which are impurities, through oxidation.
• a reduction process for iron ore and the like require approximately 500 kg of carbon source to produce 1 ton of molten pig iron, generating approximately 2 tons of CO 2 gas.
• high-grade iron ore with a low P concentration is predicted to be depleted in the future, and it will be thus required to produce molten steel using, as a raw material, reduced iron produced using low-grade iron ore with a high P concentration.
• the P concentration in iron ore used for the current blast furnace method is 0.05 to 0.10 mass % (or 0.10 to 0.15 mass % when converted into the P concentration in reduced iron), and the P concentration is predicted to further increase in the future.
• Such a P concentration is 5 to 10 times or more the P concentration in the above-mentioned reduced iron produced using high-grade iron ore with a low P concentration.
• Patent Literature 1 proposes a dephosphorization refining flux for use in an arc furnace for removing phosphorus in molten steel to achieve a low phosphorus concentration in a relatively short time with the arc furnace alone, the flux containing calcium oxide as the main ingredient, and also containing 5 to 15 mass % of aluminum oxide and 25 to 35 mass % of iron oxide, with the balance being unavoidable impurities.
• Patent Literature 2 proposes a method of removing phosphorus by bringing iron ore, titanium-containing iron ore, nickel-containing ore, chromium-containing ore, or a mixture containing such ores, each having a CaO content of 25 mass % or less and a ratio of CaO/(SiO 2 +Al 2 O 3 ) of 5 or less, as the main ingredient into contact with a gas selected from the group consisting of Ar, He, N 2 , CO, H 2 , and hydrocarbon, or a mixture gas thereof at a temperature of 1600° C. or higher.
• a gas selected from the group consisting of Ar, He, N 2 , CO, H 2 , and hydrocarbon, or a mixture gas thereof at a temperature of 1600° C. or higher.
• Patent Literature 3 proposes a method including crushing iron ore with a high P concentration to have a size of 0.5 mm or less; adding water thereto to achieve a pulp concentration of approximately 35 mass %; adding H 2 SO 4 or HCl to a solvent to allow a reaction to take place at a pH of 2.0 or less, thereby decomposing and eluting phosphorus minerals; collecting a magnetically attracted substance such as magnetite by magnetic sorting to separate therefrom SiO 2 , Al 2 O 3 , and so on, which are magnetically non-attracted substances, as slime by sedimentation separation; and also adding slaked lime or quicklime to thereby neutralize P eluted into the solution at a pH in the range of 5.0 to 10.0 to separate and recover P as calcium phosphate.
• Patent Literature 1 is based on the premise of using an iron source with a low P concentration, such as scrap. Specifically, 350 g of flux is added to 7000 g of molten steel to reduce the P concentration in the molten steel from 0.020 mass % to 0.005 mass %. Assuming that the P concentration in reduced iron is 0.15 mass %, the amount of flux required to reduce the P concentration to 0.01 mass %, which is approximately the same level in a steel product, is 230 kg per 1 ton of molten steel. This results in a high proportion of the flux volume in the arc furnace, which is problematic in that the amount of molten steel processed would decrease, reducing production efficiency.
• the treatment temperature is 1600° C. or higher.
• the specification of Patent Literature 2 describes that “the temperature is preferably 1800° C. or higher to allow more effective dephosphorization, and such a high temperature range is difficult to achieve by an ordinary heating method, but can be achieved by using a plasma arc or high-frequency induced plasma, for example.” Therefore, the method requires more energy, and thus is not suitable for large-scale dephosphorization, which is problematic.
• Patent Literature 3 The method disclosed in Patent Literature 3 is problematic in that it involves wet processing using acid, requiring a long time and high cost to dry the collected magnetically attracted substance for use as the main raw material.
• the method is also problematic as it requires a long time and high cost to crush iron ore into particles of 0.5 mm or less in advance.
• the present invention has been made in view of the circumstances described above and aims to provide a technology capable of efficiently producing grained iron with a low P concentration even when reduced iron obtained from low-grade iron ore with a high P concentration is used as a raw material.
• a method for producing grained iron according to the present invention that advantageously solves the above-described problems includes a first step of melting reduced iron to produce primary molten iron, a second step of separating the primary molten iron from slag, a third step of subjecting the primary molten iron separated from the slag to dephosphorization to produce secondary molten iron, and a fourth step of solidifying the secondary molten iron in a grain state to form grained iron, characterized in that in the third step, the dephosphorization is performed by supplying an oxygen source and a CaO source to the primary molten iron, and a temperature of the secondary molten iron at the end of the dephosphorization is set to a temperature of the primary molten iron at the start of the dephosphorization or lower.
• Grained iron according to the present invention which advantageously solves the foregoing problems is produced from reduced iron with a P concentration of 0.050 mass % or more as a raw material, characterized in that a P concentration of the grained iron is 0.030 mass % or less, and grain size of the grained iron is in the range of 1 mm to 50 mm inclusive.
• the method for producing grained iron according to the present invention can efficiently produce grained iron with a low P concentration from reduced iron obtained from low-grade iron ore with a high P concentration. Furthermore, the grained iron according to the present invention satisfies the P concentration required for most steel products, i.e., 0.030 mass % or less. Thus, molten iron with a P concentration corresponding to the level in a steel product can be obtained simply by re-melting the grained iron according to the present invention.
• the grained iron is produced by solidifying molten iron into a grained form once after the dephosphorization, it is possible to produce iron and steel not in a large-scale iron-making plant but in such a manner that a plant for producing an iron source is separated from a place where iron source is demanded.
• a production method can be considered in which a process up to the production of dephosphorized grained iron is performed in a raw material-producing country, and iron or steel is produced in an iron steel-producing country, using the dephosphorized grained iron as a raw material.
• the degree of freedom of facilities used for the transport, storage, and supply can be increased by setting the grain size of grained iron in the range of 1 to 50 mm.
• a risk such as bridging in a feed hopper can be reduced.
• Reduced iron produced using iron ore as a raw material has different properties, such as a metallization rate and composition, depending on the brand of the iron ore used, the type and unit consumption of a raw material composition adjusting agent to be mixed, the type and unit consumption of a reducing agent, a reduction temperature, and a scheme adopted for a facility for producing the reduced iron.
• Table 1 shows examples of the ingredient compositions of reduced iron.
• the P concentration converted to the P concentration in molten iron, which is obtained by dividing the P concentration by the T.Fe (total iron) concentration is 0.057 to 0.152 mass %.
• the inventors have arrived at a process of producing grained iron by melting reduced iron once to obtain molten iron, and also removing at least a part of the slag derived from gangue, and then supplying an oxygen source and a lime source to the obtained molten iron to effect dephosphorization, and further solidifying the dephosphorized molten iron into a grained form.
• reduced iron is heated and melted in an electric furnace to produce primary molten iron.
• the reduced iron to be used herein may be the one transferred as is at a high temperature from an adjacent plant for producing reduced iron, for example.
• reduced iron that has been once cooled to room temperature may also be used.
• the electric furnace may be an arc furnace, submerged arc furnace, or induction melting furnace.
• the thermal energy to be supplied in the first step to heat and melt the reduced iron, which is a solid iron source can be not only electrical energy but also, supplementally, the combustion heat of gaseous fuel such as a natural gas or a propane gas, liquid fuel such as heavy oil, or combustible solid such as coal or metallic Al or Si, for example. Such energy is preferably renewable from the perspective of reducing CO 2 emissions.
• slag which is a gangue portion of the reduced iron, and the primary molten iron are separated from each other.
• the molten metal is tapped into a vessel for transport and then transported to a facility for performing dephosphorization.
• a CaO source is added to produce slag for dephosphorization.
• at least a part of the slag containing a large amount of SiO 2 produced with the melting of the reduced iron may be carried over.
• the slag may also be removed from a vessel for heating and melting the reduced iron used in the first step, for example, by means of a slag dragger.
• the molten metal is subjected to dephosphorization to produce secondary molten iron.
• a dephosphorization reaction requires an oxygen source and a CaO source as represented by the following Expression (1).
• a pure oxygen gas is normally used as the oxygen source for dephosphorization.
• the inventors have come to the conclusion that it is advantageous to perform dephosphorization at a low temperature, since a dephosphorization reaction is an exothermic reaction, and also to reduce the temperature of the molten iron within the range that does not adversely affect dephosphorization, taking into account that the resultant is solidified to form grained iron in the following step.
• the inventors have found that sufficient dephosphorization can be achieved while cooling the molten iron, by supplying air or an iron oxide source such as iron ore or mill scale, as the oxygen source.
• air When air is used, heat removal proceeds as sensible heat of a nitrogen gas contained in the air, achieving a better cooling effect than when a pure oxygen gas is used.
• an iron oxide source when an iron oxide source is used, an endothermic reaction occurs as the iron oxide source is reduced to form metallic Fe, or heat absorption occurs as a molten slag is formed in the form of iron oxide, achieving a better cooling effect than when a pure oxygen gas is used.
• an inert gas is preferably blown into the molten iron to agitate it.
• the inert gas is preferably blown into the molten iron via a porous plug disposed at the bottom of the furnace or by immersing an injection lance in the molten iron.
• slag basicity which is the ratio of the CaO concentration (% CaO) to the SiO 2 concentration (% SiO 2 ) on a mass basis, is preferably in the range of 1.0 to 4.0.
• the slag basicity is adjusted based on the amount of slag containing a large amount of SiO 2 that is carried over to the second step, and the type and the amount of the CaO source added. It is also possible to add a SiO 2 source, such as silica stone or ferrosilicon, and a CaO source, such as quicklime, as appropriate.
• the slag basicity is low, the amount of phosphorus to be removed in dephosphorization will be small. Meanwhile, if the slag basicity is high, a part of the slag will solidify and thus become attached to a refractory when the temperature of the molten iron drops. This makes it difficult to remove the slag after dephosphorization and causes problems such that an abnormal reaction may occur the next time molten iron is charged, or the residual slag may be mixed into the produced slag, causing the composition to fall out of range.
• the secondary molten iron after the dephosphorization is solidified into a grained form to obtain grained iron.
• a method for producing grained iron include a method of flowing down molten iron subjected to dephosphorization to cause it to collide with a surface plate of a refractory, and a method of causing water to collide with the molten iron, which has flowed out, to obtain molten iron droplets, and then dropping the molten iron droplets into a water-flow control vessel to obtain solidified grained iron.
• the temperature of the molten iron decreases while the molten iron is being transported after the dephosphorization to be supplied to a grained iron producing apparatus. If the temperature of the molten iron after the dephosphorization is too low, part of the molten iron in the vessel will solidify before the molten iron is entirely supplied to the grained iron production apparatus, resulting in reduced production yields. Meanwhile, if the temperature of the molten iron after the dephosphorization is high, the heat load when the molten iron is solidified by the grained iron production apparatus will increase, increasing the amount of cooling water to be used, so that the productivity may decrease due to the cooling rate, or the waiting time until the temperature of the molten iron decreases and grained iron is obtained may become long.
• the temperature T f of the molten iron after the dephosphorization is set to the temperature T i of the molten iron at the start of the dephosphorization or lower, from the viewpoint of increasing productivity.
• the temperature T f at the end of the dephosphorization is set higher than the solidifying temperature T m of the secondary molten iron at the end of the dephosphorization by 20° C. or more, the molten iron can be supplied to the grained iron producing apparatus in a high yield, which is preferable.
• the solidifying temperature T m (° C.) may be determined by either of the following methods. First, it may be directly measured as the solidifying temperature of a sample. Alternatively, it can be a temperature read from a liquidus temperature in an Fe—C state diagram, based on the C concentration in the molten iron subjected to dephosphorization that is estimated from past records of operation (the C concentration and the temperature before dephosphorization, and the type and supply conditions of the oxygen source).
• the grained iron producing apparatus includes a granulation device which forms the molten iron into droplets, and a water-flow control vessel which is disposed at a position for receiving the droplets and accommodates cooling water. At least one cooling water pipe which supplies cooling water is connected to the water-flow control vessel into which the molten iron is dropped to solidify. As the cooling water is discharged from the cooling water pipe to form a water flow, the formation of a stagnation region of the cooling water within the vessel is suppressed. This can suppress a local temperature rise of the cooling water and efficiently cool grained iron to suppress the fusion of grained iron caused by insufficient cooling of grained iron.
• the water-flow control vessel has an inclined surface which is inclined such that the horizontal cross-sectional area of the vessel becomes narrower in a downward direction, and a discharge port is provided below the inclined surface. Setting the inclination angle of the inclined surface to be the angle of repose of grained iron in water or more allows grained iron to be directed to the discharge port without accumulation of grained iron on the inclined surface.
• the effect of diluting the P concentration in accordance with the proportion of the grained iron used can be achieved. This can reduce the load in dephosphorization and ease restrictions on the raw materials to be used in the blast furnace and converter.
• the grained iron obtained in this embodiment is used as an iron source in an electric furnace, blast furnace, or converter, there is a range of grain sizes that are convenient to work with. To obtain the desired grain sizes, it is preferable to adjust the flowing-down speed in the tundish. It is also preferable to perform classification as required. Typically, grained iron with a grain size in the range of 1 to 50 mm is convenient to use. When grained iron with a grain size of less than 1 mm is included, there is a higher possibility of clogging a conveyor for transport or bridging in a hopper. Therefore, it is preferable to perform classification so as to obtain grained iron with a grain size of 1 mm or more for use.
• the grain size in the range of 1 to 50 mm may include particles on a sieve with an opening of 1 mm to particles that have passed through a sieve with an opening of 50 mm.
• the reduced iron A shown in Table 1 was melted in an electric furnace with a capacity of 250 tons, and, after adjusting the temperature of the resultant, transferred to a ladle.
• a ladle was transferred to the ladle together with the molten iron, and the rest of the slag was transferred to a slag vessel.
• the ladle was transferred to a dephosphorization facility to perform dephosphorization while changing the types and amounts of an oxygen source and a lime source supplied.
• the dephosphorization facility included a gas top-blowing lance, an auxiliary material feeding hopper, and a bottom-blowing porous plug.
• the gas top-blowing lance was capable of supplying gas containing pure oxygen or air at a rate of approximately 1 Nm 3 /minute per 1 ton of molten iron.
• the bottom-blowing porous plug can supply gas.
• a pure Ar gas was supplied at a rate of approximately 0.1 Nm 3 /minute per 1 ton of molten iron.
• the melting temperature in the electric furnace was adjusted to allow the temperature of the molten iron before dephosphorization to be approximately 1590° C.
• “Before dephosphorization” refers to the time before the gas top-blowing lance is lowered, while “after dephosphorization” refers to the time when the gas top-blowing lance has been completely raised after the dephosphorization.
• temperature measurements and sampling were conducted using a sublance. The obtained samples were cut and polished and subjected to an emission spectrochemical analysis to evaluate the C concentration [C] and the P concentration [P] in the molten iron from calibration curves determined in advance. It was possible to measure the solidifying temperature of the molten metal at the timing when the temperature measurement and sampling were performed using the sublance, and the solidifying temperature T m of the molten iron subjected to the dephosphorization was actually measured.
• the start of the dephosphorization was defined as when the gas top-blowing lance started to be lowered. After the top-blowing lance reached a predetermined height, the supply of an oxygen gas source and the addition of auxiliary materials were started. The dephosphorization was terminated when the supply of predetermined amounts of oxygen gas source and auxiliary materials was completed and the top-blowing lance was raised to a standby position. The duration of the period was determined as a processing time t f (minutes).
• the ladle was tilted to remove the slag on the molten iron with a slag dragger. Part of the removed slag was collected and subjected to a chemical analysis.
• the ladle was lifted and tilted using a crane to transfer the molten iron to the tundish.
• the molten iron was caused to flow down from the tundish so as to collide with a surface plate of a refractory, and the resulting molten iron droplets were dropped into the water-flow control vessel and solidified to produce grained iron.
• the grain sizes of the obtained grained iron ranged from 0.1 to 30 mm.
• the grain size distributions were: +0.1 mm to ⁇ 1 mm: 17.2 mass %, +1 mm to ⁇ 10 mm: 31.3 mass %, +10 mm to ⁇ 20 mm: 38.8 mass %, and +20 mm to ⁇ 30 mm: 12.7 mass %.
• “+N to ⁇ M” means particles on a sieve with an opening of N to particles that have passed through a sieve with an opening of M.
• Table 2 shows, as Test Nos. 1 to 5, the temperatures T i and T f (° C.), the C concentrations [C] i and [C] f (mass %), and the P concentrations [P] i and [P] f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and the amounts of the oxygen source and the CaO source supplied; the processing time t f (minutes); and the basicity of the slag after the process ((% CaO)/(% SiO 2 ), i.e., the ratio of the CaO concentration (% CaO) to the SiO 2 concentration (% SiO 2 ) on a mass basis; hereinafter referred to as C/S).
• the temperature T f of the molten iron after the process was lower than the temperature T i of the molten iron before the process, and the P concentration [P] f after the process was sufficiently lowered.
• the temperature T f after dephosphorization increased higher than the temperature T i before dephosphorization, and consequently the P concentration [P] f after the process was high, and a waiting time was caused during the grained iron production step, resulting in decreased productivity.
• Test No. 4 compared with Test Nos. 1 to 3
• the temperature T f of the molten iron after the process decreased to reduce the P concentration [P] f sufficiently.
• Table 3 shows the temperatures T i and T f (° C.), the C concentrations [C] i and [C] f (mass %), and the P concentrations [P] i and [P] f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and amounts of the oxygen source and CaO source supplied; the processing time t f (minutes); and the basicity C/S of the slag after the process, as Test Nos. 6 to 12.
• Test No. 11 the basicity C/S of the slag was low compared with Test Nos. 6 to 10, and thus the P 10 concentration after the process was high. Meanwhile, in Test No. 12, the basicity C/S of the slag was high, and the solidification of the slag was confirmed.
• the reduced iron A shown in Table 1 was melted with anthracite in an electric furnace with a capacity of 250 tons to produce molten iron with a C concentration of approximately 2.0 mass %. After adjusting the temperature of the molten iron, the molten iron was transferred to a ladle, where dephosphorization and a production of grained iron were conducted by a method similar to those of Examples 1 and 2.
• Table 4 shows the temperatures T i and T f (° C.), the C concentrations [C] i and [C] f (mass %), and the P concentrations [P] i and [P] f (mass %) of the molten iron before and after the dephosphorization, respectively; the types and amounts of the oxygen source and CaO source supplied; the processing time t f (minutes); and the basicity C/S of the slag after the process, as Test Nos. 13 to 19.
• the basicity C/S of the slag in Test No. 18 was low compared with Test Nos. 13 to 17, and thus the P concentration [P] f after the process was high. Meanwhile, in Test No. 19, the basicity C/S of the slag was high, and the solidification of the slag was confirmed.
• the grained iron produced in each of Test Nos. 8 to 10, 12, 14 to 17, and 19 were found to have a P concentration of 0.030 mass % or less.
• the reduced iron of each process was melted in an electric furnace, the obtained molten iron was found to have a P concentration of 0.030 mass % or less.
• Such a P concentration has reached a level required of a steel product, thus requiring no additional dephosphorization.
• the grained iron obtained in each of Test Nos. 8 to 10, 12, 14 to 17, and 19 could be used in an electric furnace, a blast furnace, or a converter without any problem.
• the unit “t” of a mass represents 103 kg.
• the symbol “N” added to the unit “Nm 3 ” of a volume represents the standard state of gas.
• Symbol [M] in a chemical formula represents that an element M is melted in molten iron or reduced iron.
• the method for producing grained iron and grained iron of the present invention it is possible to efficiently produce grained iron with a low P concentration even when reduced iron obtained from low-grade iron ore with a high P concentration is used as a raw material.
• only remelting the grained iron according to the present invention can obtain molten iron with a P concentration corresponding to the level in a steel product.
• the present invention is industrially advantageous.

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• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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• 2023
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