METHOD FOR PRODUCING LOW-CARBON FERROMANGANESE

Abstract

A method for producing low-carbon ferromanganese capable of achieving a high Mn yield. In producing low-carbon ferromanganese by blowing an oxidizing gas from a top-blowing lance onto a bath face of high-carbon ferromanganese molten metal accommodated in a reaction vessel provided with a top-blowing lance and bottom-blowing tuyere to perform decarburization, the slag composition during the blowing is adjusted so that a value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) on a mass basis in the slag composition is not less than 0.4 but not more than 5.0. Also, agitation is performed under a condition that an agitation power density ε of an agitation gas blown through the bottom-blowing tuyere is not less than 500 W/t.

Claims

1. A method for producing low-carbon ferromanganese comprising blowing an oxidizing gas onto a bath face of high-carbon ferromanganese molten metal contained in a reaction vessel provided with a top-blowing lance and bottom-blowing tuyere, from the top-blowing lance for decarburization, wherein a slag composition during the blowing is adjusted so that a value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) on a mass basis in the slag composition is not less than 0.4 but not more than 5.0.

2. The method for producing low-carbon ferromanganese according to claim 1, wherein an agitation gas is blown from the bottom-blowing tuyere to have an agitation power density of not less than 500 W/t.

3. The method for producing a low-carbon ferromanganese according to claim 1, wherein an auxiliary material containing MgO is added before the start of the blowing or during the blowing.

4. The method for producing a low-carbon ferromanganese according to claim 2, wherein an auxiliary material containing MgO is added before the start of the blowing or during the blowing.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0034] FIG. 1 is a schematic view of an equipment used in an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

[0035] FIG. 1 shows an example of effective equipment for performing the method of the invention. There will be described the method of decarburization-refining of high-carbon ferromanganese (HCFeMn) with reference to FIG. 1 below. FIG. 1 shows that melted high-carbon ferromanganese molten metal 2 is charged into a reaction vessel 1 as an example of a top-bottom blown converter. An oxidizing gas is blown from a top-blowing lance 3 onto the bath face of the molten metal 2 in the vessel. Note that the oxidizing gas means pure oxygen gas or oxygen mixed gas. Whereas, a non-oxidizing gas is blown into the molten metal 2 from a bottom-blowing tuyere 4, to which a pipe 5 for introducing the non-oxidizing gas is connected. In the example shown in FIG. 1, flow control valves 8 are provided both in a pipe 6 and pipe 7 for introducing the non-oxidizing gas and the oxygen gas, respectively, into the top-blowing lance 3. Note that various types of additives 9 can be charged from a furnace throat during the blowing. The molten metal 2 has slag 10 formed thereon. The additive 9 can use an auxiliary material containing MgO, ferromanganese as a cooling material, and the like. A Laval nozzle is preferably used as the top-blowing lance 3. When a plurality of nozzles are used, they are preferably arranged in rotation symmetry to the axis of the lance. Further, the top-blowing lance 3 is preferable to use a multiport lance, leading to a wider hot spot area than a single port lance and efficient blowing of oxygen to the molten metal, and is thus suitable for the mass production.

[0036] In the method for producing low-carbon ferromanganese according to the invention, the molten metal 2 of high-carbon ferromanganese is first charged into the reaction vessel 1. From before the charging of the molten metal 2 to during the refining thereof, a required amount of non-oxidizing gas is blown into the molten metal 2 from the bottom-blowing tuyere 4 to agitate the molten metal 2. Thereafter, the top-blowing lance 3 is descended from above to spray an oxidizing gas onto a bath face of the molten metal 2 to thus start decarburization blowing. If necessary, an auxiliary material containing MgO may be added before the start of the blowing.

[0037] The oxidizing gas blown from the top-blowing lance 3 can use an oxygen gas or oxygen mixed gas obtained by mixing oxygen gas with not more than 30 vol % non-oxidizing gas; Ar is preferable as the non-oxidizing gas to be mixed. From the viewpoint of securing the temperature of the hot spot, the oxidizing gas for top-blowing is preferably oxygen mixed gas containing not more than 10 vol % non-oxidizing gas, more preferably pure oxygen gas. Whereas, the non-oxidizing gas to be blown from the bottom-blowing tuyere is preferably Ar, CO, CO.sub.2, or a mixture gas thereof from the viewpoint of efficient agitation without increasing nitrogen concentration in the molten metal.

[0038] According to the invention, the slag composition is adjusted so that a value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) on a mass basis in the slag composition during blowing is not less than 0.4 but not more than 5.0 in the above operation. If necessary, an auxiliary material such as an alloy, quicklime, dolomite or the like, Mn ore, blast furnace slag, and so on may be added properly for the adjustment of the composition of the slag 10. When the value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) is less than 0.4 or exceeds 5.0, a solid phase rate of the slag 10 increases. As a result, the viscosity of the slag increases to lower the fluidity thereof, failing to reduce MnO in the slag 10 into the molten metal efficiently, and hence such a value is not preferable. A more preferable lower limit is not less than 1.0 while a more preferable upper limit is not more than 3.0. The composition of the slag during blowing is the one obtained when the slag is sufficiently formed, and it can be confirmed by analysis at the end of the blowing.

[0039] The activity of MnO represented with a.sub.MnO can be kept at a high level by adjusting the slag composition such that MnO/(MnO+CaO+Al.sub.2O.sub.3+MgO+SiO.sub.2) on a mass basis is not less than 0.6, whereby MnO in the slag 10 can be reduced into the molten metal at a higher efficiency. The upper limit is not particularly limited, but is less than 1.0.

[0040] In the above operation, it is preferable to feed bottom-blowing gas to the molten metal 2 from the tuyere 4 under a condition that an agitation power density ε of the molten metal 2 represented by the following equation (5) is not less than 500 W/t, to accelerate the decarburization reaction during the blowing. The reason is that conducting agitation of the molten metal at a proper agitation power density promotes the slag-metal reaction, enabling Mn oxide (MnO) in the slag 10 to be recovered in the molten metal. More preferably, the agitation power density is not less than 600 W/t. Whereas, even if a larger amount of bottom-blowing gas is blown, the bottom-blowing gas effectively contributing to the agitation of the molten metal is usually decreased due to blow-out, so that a maximum value of the agitation power density is about 1000 W/t.

ε=6.183×(Q×T.sub.l/(60×W))×[ln {1+h/(1.02×10.sup.−4×(101325×P))}+{1−(T.sub.g/T.sub.l)}]  (5)

Note that, ε represents the agitation power density (W/t) of the bottom-blowing gas; Q represents the flow rate (Nm.sup.3/h) of the bottom-blowing gas; W represents the amount (t) of the ferromanganese molten metal; T.sub.l represents the temperature (° C.) of the ferromanganese molten metal; T.sub.g represents the temperature (° C.) of the bottom-blowing gas; h represents the bath depth (distance from the bath face to the bottom of the reaction furnace at rest) (m); P represents the atmospheric pressure (1 atm).

[0041] The blowing of the oxidizing gas from the top-blowing lance 3 is preferably conducted such that the flow velocity of the oxidizing gas when arriving at the bath face calculated by the following equations (6) to (9) is not less than 70 m/s but not more than 150 m/s. This is due to the fact that the operation within the above range enables blowing while suppressing the scattering of the molten metal without oxygen being blocked by the vapor of Mn (fume), thus improving the decarburization efficiency of oxygen to provide a high Mn yield. More preferably, blowing is operated so that the flow velocity when arriving at the bath face falls within a range of not less than 80 m/s to not more than 130 m/s.

F.sub.o2=0.456.square-solid.n.square-solid.d.sup.2.square-solid.(P.sub.0/0.97)  (6)

U.sub.0=740{1−(P.sub.e/P.sub.0).sup.2/7}.sup.1/2  (7)

U/U.sub.0=D/2CL  (8)

C=0.016+0.19/((P.sub.0/0.97)−1.034)  (9)

Note that, F.sub.O2 represents the flow rate (Nm.sup.3/h) of the oxidizing gas from the top-blowing lance; n represents the nozzle number (nozzles) of the top-blowing lance; d represents the throat size (mm) of the nozzle of the top-blowing lance; P.sub.0 represents the pressure (atm) of the oxidizing gas at the nozzle inlet of the top-blowing lance; P.sub.e represents the pressure (atm) of the oxidizing gas at the nozzle outlet of the top-blowing lance; U.sub.0 represents the blowing velocity (m/s) of the oxidizing gas from the top-blowing lance; U represents the flow velocity (m/s) of the oxidizing gas when arriving at the bath face from the top-blowing lance; L represents the lance height (distance from the outlet of the nozzle in the top-blowing lance to the bath face at rest) (mm); D represents the outlet size (mm) of the nozzle of the top-blowing lance; C represents a constant (-) that represents the spreading of the oxidizing gas jet.

[0042] From the viewpoint of preventing wear damage of the refractory, suppressing evaporation of Mn, and preventing slowdown of the decarburization rate, it is preferable to operate at the temperature of the ferromanganese molten metal T.sub.l of not higher than 1700° C. when the carbon concentration of the molten metal [C] is not less than 2.0 mass % and of not higher than 1750° C. when [C] is not less than 1.5 mass % but less than 2.0 mass %. In order to maintain the molten metal temperature within the above range, it is effective to add an auxiliary material such as alloy, quicklime, dolomite or the like, Mn ore, slag, and so on as a cooling material 9 during the decarburization refining, if necessary. In this case, if the slag volume is increased beyond suppression of Mn evaporation, the migration of Mn into the slag is increased, causing a decrease in the Mn yield. Therefore, the cooling material is preferably crushed scrap of FeMn, more preferably MCFeMn or LCFeMn. However, the control of the operating temperature using such crushed scrap is not preferable from the viewpoint of accelerating the decarburization reaction, because the molten metal is locally cooled by the addition of the cooling material. Therefore, when using the crushed scrap to control the molten metal temperature, it is desirable to decrease the use amount as low as possible. Also, adding the auxiliary material containing MgO before the start of the blowing or during the blowing enables the erosion of the bricks to be decreased, leading to an increase in the service life of the refractory. The auxiliary material containing MgO may include MgO ball (material formed by sintering dolomite or magnesite, pulverized, and then fixed with cement) and magnesite (ore composed mainly of magnesium carbonate).

[0043] The top-blowing lance is uplifted to stop the blowing of the oxidizing gas after decarburization is performed up to a given carbon concentration. After the uplifting of the top-blowing lance 3, it is preferable to recover Mn oxide (MnO) in the slag by adding reducing material such as FeSi, SiMn, or the like while agitating by the bottom-blowing gas.

EXAMPLE

[0044] This example is a case where 25 t of high-carbon ferromanganese (HCFeMn) molten metal was charged into a top and bottom blown type cylindrical refining furnace having an inner diameter of about 2.3 m to conduct decarburization refining. The used HCFeMn corresponds to the case No. 2 shown in Table 1 (Mn: 73 mass %, C: 6.9 mass %), in which a temperature immediately after the charging was 1334 to 1341° C. In the operation (blowing), pure O.sub.2 was blown on the molten metal from a top-blowing lance, while Ar was blown therein from a bottom-blowing tuyere for agitation. The oxygen flow rate was 40 Nm.sup.3/min from the start of the refining to the end thereof. In the blowing, 500 kg of crushed scrap of MCFeMn (Mn: 80 mass %, C: 1.5 to 2.0 mass %) and 450 kg of crushed scrap of LCFeMn (Mn: 80 mass %, C: 0.5 to 1.0 mass %) were added. In this operation, lime, silica, band shale, and dolomite were added to change the slag composition for the decarburization refining of the high-carbon ferromanganese molten metal, if required. Table 2 shows the result. Note that the blowing was stopped when the carbon concentration [C] in the ferromanganese molten metal reached 0.5 mass % as an endpoint.

TABLE-US-00001 TABLE 1 Component composition (mass %) Type Symbol Mn C Si P S High-carbon 0 FMnH0 78 to 82 <7.5 <1.2 <0.4 <0.02 ferromanganese 1 FMnH1 73 to 78 <7.3 <1.2 <0.4 <0.02 (HCFeMn) 2 FMnH2 73 to 78 <7.0 <3.0 <0.4 <0.02 Medium-carbon 0 FMnM0 80 to 85 <1.5 <1.5 <0.4 <0.02 ferromanganese 1 FMnM1 75 to 80 <2.0 <2.0 <0.4 <0.02 (MCFeMn) Low-carbon 0 FMnL0 80 to 85 <1.0 <1.5 <0.35 <0.02 ferromanganese 1 FMnL1 75 to 80 <1.0 <1.5 <0.4 <0.02 (LCFeMn) Silicon 0 SiMn0 65 to 70 <1.5 20 to 25 <0.3 <0.05 manganese 1 SiMn1 65 to 70 <2.0 16 to 20 <0.3 <0.02 2 SiMn2 60 to 65 <2.0 16 to 20 <0.3 <0.03 3 SiMn3 60 to 65 <2.5 14 to 18 <0.3 <0.03 Based on JIS G2301: 1998 and JIS G2304: 1998

TABLE-US-00002 TABLE 2 Agitation (CaO + MgO)/ power (Al.sub.2O.sub.3 + SiO.sub.2) density Mn yield No. [—] ε[W/t] η.sub.Mn [%] Remarks 1 0.1 634 69 Comparative Example 2 0.2 634 70 Comparative Example 3 0.3 634 75 Comparative Example 4 0.4 634 80 Inventive Example 5 0.6 634 81 Inventive Example 6 0.8 634 82 Inventive Example 7 1.0 634 84 Inventive Example 8 1.5 634 85 Inventive Example 9 2.0 634 85 Inventive Example 10 2.5 634 86 Inventive Example 11 3.0 634 85 Inventive Example 12 3.5 634 84 Inventive Example 13 4.0 634 83 Inventive Example 14 4.5 634 82 Inventive Example 15 4.8 634 81 Inventive Example 16 5.2 634 75 Comparative Example 17 5.5 634 72 Comparative Example 18 6.0 634 70 Comparative Example 19 2.0 423 80 Inventive Example 20 2.0 508 81 Inventive Example 21 2.0 761 89 Inventive Example

[0045] In this example, the Mn yield is defined by the following equation (10):

η.sub.Mn=W.sub.1/(W.sub.2+W.sub.3+W.sub.4)×100  (10)

Note that, η.sub.Mn represents the Mn yield (0%); W.sub.1 represents the mass (kg) of Mn in a FeMn product; W.sub.2 represents the mass (kg) of Mn in HCFeMn molten metal; W.sub.3 represents the mass (kg) of Mn added as Mn-containing alloy; W.sub.4 represents the mass of Mn added as Mn oxide (kg).

[0046] As seen from the result of the decarburization refining shown in Table 2, the Mn yield is at a higher level when the value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) is not less than 0.4 but not more than 5.0 (Nos. 4 to 15 and 19 to 21). On the other hand, when the value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) is less than 0.4 (Nos. 1 to 3) or exceeds 5.0 (Nos. 16 to 18), the Mn yield η.sub.Mn is at a low level, seemingly because MnO in the slag is not reduced sufficiently. When the cases having the value of (CaO+MgO)/(Al.sub.2O.sub.3+SiO.sub.2) of 2.0 are compared, the Mn yield η.sub.Mn is at a higher level in some cases (Nos. 9, 20, and 21) when the agitation power density ε of the bottom-blowing gas is not less than 500 W/t than in another case (No. 19) when the agitation power density ε is less than 500 W/t. This is considered due to the fact that when the molten metal is agitated at a proper agitation power density ε, the slag-metal reaction can be promoted to reduce Mn oxide in the slag in a good efficiency for recovery.

[0047] In this description, non-SI unit is converted to SI unit by the following conversion numeral:

1 atm=101325 Pa

INDUSTRIAL APPLICABILITY

[0048] The technique proposed in the method for producing low-carbon ferromanganese according to the invention can be expanded, for example, in the field of other usual steel-making refining technique.

REFERENCE SIGNS LIST

[0049] 1 reaction vessel [0050] 2 molten metal [0051] 3 lance [0052] 4 tuyere [0053] 5 pipe for non-oxidizing gas [0054] 6 pipe for non-oxidizing gas [0055] 7 pipe for oxygen [0056] 8 flow control valve [0057] 9 additive [0058] 10 slag