Ferronickel manufacturing method

Abstract

To maintain quality of ferronickel appropriate even when calcium oxide is used as a desulfurizing agent for removing sulfur from rough ferronickel melt in a pyrometallurgical method for producing ferronickel.SOLUTION: A manufacturing method of ferronickel comprises: a reduction melting step S1 in which nickel oxide ore is reduced in a melting furnace to obtain a crude ferronickel melt having a nickel grade of 14 mass% or more and a silicon grade of 3% or less; and a desulfurization step S2 of charging a desulfurization agent in the crude ferronickel melt to remove sulfur, in the desulfurization step S2, a silicon quality control treatment st1 wherein calcium oxide is used as the desulfurization agent to maintain a Si / CaO ratio in the crude ferronickel melt at a predetermined control value is performed in advance the desulfurization step S2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing ferronickel. [Background technology]

[0002] Ferronickel is an alloy primarily composed of iron and nickel, and is used as a raw material for stainless steel and special steel. Ferronickel is generally produced by a pyrometallurgical process using nickel oxide ore as a raw material (see Patent Document 1).

[0003] In the above-mentioned pyrometallurgical smelting method for producing ferronickel, nickel oxide ore is heated and dried, and the cinder obtained by partial reduction is supplied to an electric furnace for melting and reduction, thereby recovering the target metals nickel (Ni) and iron (Fe) as a "crude ferronickel molten liquid" which is an Fe-Ni alloy, in a "reduction melting step," and sulfur (S) contained as an impurity is removed from the "crude ferronickel molten liquid" by treatment with a desulfurizing agent in a "desulfurization step." The refined ferronickel that has undergone the desulfurization step is often further cast in a casting step and shipped as shot-shaped (flake-shaped) ferronickel cast pieces (see Patent Document 1).

[0004] The typical composition of the above nickel oxide ore is Ni: about 2%, Fe: about 22%, MgO: about 16%, SiO 2: The content of the crude ferronickel molten metal is about 35%. In the production of ferronickel by the pyrometallurgical refining method, the "crude ferronickel molten metal" recovered as metal in the above-mentioned "reduction melting process" is mainly composed of iron and contains nickel in a proportion of 14% by mass to 25% by mass, and also contains impurities such as sulfur. In this process, the residue other than the "crude ferronickel molten metal" is separately recovered as MgO-SiO2-FeO slag.

[0005] In the above-mentioned "reduction melting process," in order to ensure the quality of ferronickel as a raw material for stainless steel, etc., it is required to sufficiently reduce the silicon (Si) content in the "crude ferronickel molten liquid" (specifically, as a general standard, the silicon content must be 3% or less). In response to this requirement, most of the SiO2 contained in the nickel oxide ore is distributed to the above-mentioned MgO-SiO2-FeO slag side in the "reduction melting process."

[0006] In the "desulfurization step" in which sulfur is further removed from the "crude ferronickel molten liquid" in which the silicon content has been sufficiently reduced in the "reduction melting step," calcium carbide (CaC2) has conventionally been mainly used as a desulfurization agent (see Patent Document 2).

[0007] Here, the price of calcium carbide has risen sharply in recent years, making it difficult to obtain. Therefore, it is conceivable to use calcium oxide (CaO), which is cheaper and more readily available, as a desulfurization agent instead of calcium carbide. However, because the desulfurization reaction by calcium oxide is an endothermic reaction, when calcium oxide is used as a desulfurization agent in the "desulfurization step," there is an increased risk of a decrease in the quality of the final product due to a decrease in the temperature of the "crude ferronickel molten liquid." Specifically, the risk of a decrease in quality due to a decrease in the temperature of the "crude ferronickel molten liquid" is the risk that the "crude ferronickel molten liquid" will locally solidify and deteriorate in reactivity due to contact with the desulfurization agent. Furthermore, when the above-mentioned casting step is performed, there is also a high risk that a decrease in the temperature of the "crude ferronickel molten liquid" will cause the molten liquid to settle in the trough through which the molten ferronickel flows, making casting difficult.

[0008] Since no means capable of reliably avoiding the above-mentioned risks has yet been found, calcium carbide is still widely used as a desulfurization agent in the pyrometallurgical method for producing ferronickel, and the replacement with calcium oxide has not progressed to date. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2019-39045 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-199981 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention is a process developed in view of the above circumstances, and aims to provide a method for producing ferronickel that can appropriately maintain the quality of ferronickel even when calcium oxide is used as a desulfurizing agent for removing sulfur from a "crude ferronickel molten liquid" in a pyrometallurgical process for producing ferronickel in which the silicon grade has been sufficiently reduced from nickel oxide ore containing silicon. [Means for solving the problem]

[0011] The present inventors have found that the above-mentioned problems can be solved by intentionally maintaining the silicon content of the crude ferronickel molten metal discharged from a melting furnace at a predetermined control value or higher, unlike conventional methods, and have thus completed the present invention. Specifically, the present invention provides the following.

[0012] (1) A method for producing ferronickel, in which ferronickel containing nickel as a main component is obtained from nickel oxide ore containing nickel, iron, silicon, and sulfur, comprising: a reduction-melting step in which the nickel oxide ore is reduced in a melting furnace to obtain a crude ferronickel molten liquid having a nickel content of 14% by mass or more and a silicon content of 3% or less; and a desulfurization step in which a desulfurization agent is added to the crude ferronickel molten liquid to remove sulfur, wherein calcium oxide is used as the desulfurization agent in the desulfurization step, and a silicon content adjustment treatment is carried out prior to the desulfurization step to adjust the silicon content in the crude ferronickel molten liquid so that the Si / CaO ratio in the crude ferronickel molten liquid is maintained at or above a predetermined control value.

[0013] According to the "method for producing ferronickel" in (1), in a pyrometallurgical process for producing ferronickel in which the silicon grade has been sufficiently reduced from nickel oxide ore containing silicon, even when calcium oxide is used as a desulfurizing agent for removing sulfur from the "molten crude ferronickel," a decrease in the temperature of the molten crude ferronickel can be avoided, and the quality of the ferronickel can be maintained at a good level.

[0014] (2) The method for producing ferronickel according to (1), wherein the predetermined control value of the Si / CaO ratio in the crude ferronickel molten metal is set to 0.16 or more in mass ratio. According to the "method for producing ferronickel" of (2), the above-mentioned effects of the invention of (1) can be obtained more accurately and reliably, and this makes it possible to easily carry out a higher level of quality control of ferronickel.

[0015] (3) The method for producing ferronickel according to either (1) or (2), wherein the melting furnace in which the reduction melting step is carried out is an electric furnace in which a plurality of rod-shaped electrodes are vertically movable, and the silicon grade adjustment treatment is carried out by carrying out any one or more of the following operations i) to iii): i) Raise the electrode away from the interface between the slag layer and the metal layer. ii) Increase the melting point of the slag that forms the slag layer. iii) Increasing the thickness of the slag layer.

[0016] According to the "method for producing ferronickel" in (3), the outflow of silicon (Si) components into the slag layer is suppressed by carrying out an operation (the operations i) to iii) above) for maintaining a crust layer, which is a layer made of semi-molten material in a non-equilibrium state formed at the interface between a metal layer and a slag layer in an electric furnace where nickel oxide ore is melted and reduced. This allows the Si / CaO ratio in the crude ferronickel molten metal to be optimized in a process upstream of the desulfurization process in the production of ferronickel, and makes it possible to maintain good quality of the ferronickel obtained after the desulfurization process without the need to add additional agents containing silicon. [Effects of the Invention]

[0017] According to the present invention, in a pyrometallurgical method for producing ferronickel in which the silicon grade has been sufficiently reduced from nickel oxide ore containing silicon, even when calcium oxide is used as a desulfurizing agent for removing sulfur from a "crude ferronickel molten liquid", the quality of the ferronickel can be appropriately maintained. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a process diagram showing an example of the flow of a method for producing ferronickel of the present invention. [Figure 2] FIG. 1 is a diagram schematically showing the configuration of an electric furnace, which is a specific example of a melting furnace suitable for carrying out the method for producing ferronickel of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Specific embodiments of the present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention.

[0020] <Method of manufacturing ferronickel> A widely used method for producing ferronickel is a pyrometallurgical process comprising a reduction-melting step S1 in which nickel oxide ore is reduced in a melting furnace to obtain a crude ferronickel molten liquid, and a desulfurization step S2 in which the crude ferronickel molten liquid obtained in the reduction-melting step S1 is desulfurized. The "method for producing ferronickel" of the present invention is a production method in which the essential requirements of this pyrometallurgical process are that calcium oxide is used as a desulfurization agent in the desulfurization step S2, and that a silicon quality adjustment treatment st1 is carried out as an essential treatment prior to the desulfurization step S2, in which the silicon quality in the crude ferronickel molten liquid is maintained at or above an original predetermined control value (see FIG. 1).

[0021] Incidentally, in the "method for producing ferronickel" of the present invention, as in the case of the above-mentioned conventional pyrometallurgical process (see Patent Document 1), the raw material nickel oxide ore is subjected to preliminary treatments (not shown) such as a drying step, a calcination step, and a partial reduction step before being input into the reduction melting step S1. Furthermore, after the desulfurization treatment in the desulfurization step S2, it is preferable to further carry out a casting step (not shown) in which the crude ferronickel molten metal is cast into shot form. Furthermore, since the present invention can avoid a drop in the temperature of the molten metal during the casting step, it is also a process particularly suitable for cases in which this casting step is essential.

[0022] [Reduction melting process] The reduction melting step S1 is a step in which nickel oxide ore containing nickel, iron, silicon, magnesium, sulfur, etc. is reduced in a melting furnace such as an electric furnace to produce crude ferronickel molten metal and slag. Prior to this step, preliminary treatments such as the drying step, calcination step, and partial reduction step are carried out, and the nickel oxide ore is charged into the electric furnace ore in the calcined state after these treatments.

[0023] In the "method for producing ferronickel" of the present invention, the nickel content of the crude ferronickel molten metal produced in the reduction melting step S1 is set to at least 14% by mass, preferably 20% by mass or more. Furthermore, the silicon content of the crude ferronickel molten metal is set to 3% or less in order to maintain good product quality of ferronickel as a raw material for stainless steel and the like. The crude ferronickel molten metal produced through the treatment in the reduction melting step S1 is tapped from the taphole of a melting furnace such as an electric furnace using an oxygen lance or the like, and is charged into a container such as a ladle for the next step, the desulfurization step S2. The crude ferronickel molten metal obtained in the reduction melting step S1 in this way contains iron as a main component, nickel of a predetermined content or higher, and also contains impurities such as sulfur.

[0024] Meanwhile, the iron oxide (FeO2), silicon dioxide (SiO2), and magnesium oxide (MgO) contained in the nickel oxide ore are distributed in the slag, which is produced separately from the crude ferronickel molten metal. The slag, into which most of the silicon component of the raw ore is distributed, is used as a magnesia flux for adjusting the composition in the sintering process of steel, as fine aggregate for concrete, and as a material for civil engineering works.

[0025] [Desulfurization process] In the desulfurization step S2, a desulfurization treatment is carried out in which sulfur contained as an impurity in the crude ferronickel molten liquid generated through the treatment in the reduction melting step S1 is stirred and removed together with a desulfurizing agent using a stirring device of a mechanical type, an electromagnetic induction type, or the like.

[0026] In the desulfurization step S2, first, as necessary, oxygen or the like is blown in (oxygen blowing) for the purpose of raising the temperature of the crude ferronickel molten metal in the ladle. Then, a desulfurization agent is added to the crude ferronickel molten metal in the ladle and stirred, so that the sulfur in the crude ferronickel molten metal is fixed in the slag as calcium sulfide (CaS) and separated and removed.

[0027] As described above, in conventional ferronickel smelting, calcium carbide has been widely used as a desulfurization agent for fixing sulfur in the crude ferronickel molten metal in the slag as calcium sulfide in the desulfurization step (see Patent Document 2). In contrast to this, in the "method for producing ferronickel" of the present invention, calcium oxide (CaO) is used instead of calcium carbide as the desulfurization agent. Note that the desulfurization agent used in the "method for producing ferronickel" of the present invention is not limited to one consisting of calcium oxide alone, and desulfurization agents other than calcium oxide can also be used in combination as long as they do not impair the effects of the present invention.

[0028] Here, we will explain the desulfurization reaction when calcium oxide is used as the desulfurization agent. The molecular weight of calcium oxide is 56 [g / mol], and its desulfurization reaction is an endothermic reaction as shown in the following (Equation 1). (Formula 1):S+CaO=CaS+1 / 2O 2 -264[kJ / mol]

[0029] That is, when 1 g of calcium oxide is added as a desulfurization agent, 4.71 [kJ / g] of heat is removed from the crude ferronickel molten metal, as shown in the following (Equation 2). (Formula 2):-264[kJ / mol] / 56[g / mol]=-4.71[kJ / g]

[0030] On the other hand, when calcium carbide is used as a desulfurization agent in accordance with conventional methods, the desulfurization reaction is an exothermic reaction as shown in the following formula (3). (Formula 3):S+CaC2=CaS+2C+388[kJ / mol]

[0031] From (Equation 2) and (Equation 3), it can be seen that when the same amount of sulfur (S) is fixed in slag as calcium sulfide (CaS) in the desulfurization step of a ferronickel production method, the difference in the amount of heat generated in the reaction system between when calcium carbide is used as a desulfurization agent and when calcium oxide is used is 652 [kJ / mol] (+388 [kJ / mol] - (-264 [kJ / mol])). Specifically, if 800 kg of calcium carbide is used as a desulfurization agent in the desulfurization step of a conventional ferronickel production method, and this is all replaced with calcium oxide, the molten metal temperature will decrease by 134°C. In the pyrometallurgical process for producing ferronickel, as mentioned above, a decrease in the temperature of the "crude ferronickel molten metal" carries a high risk of leading to a decrease in the quality of the final product, so calcium carbide is widely used as a desulfurization agent in conventional pyrometallurgical processes.

[0032] In contrast to this, the "method for producing ferronickel" of the present invention improves the overall configuration of the process by carrying out a "silicon grade adjustment treatment st1" as a unique treatment prior to the desulfurization step S2, thereby making it possible to avoid a decrease in the temperature of the crude ferronickel molten metal, which causes a deterioration in the quality of ferronickel, even though the process uses calcium oxide as a desulfurization agent in the desulfurization step S2. Details of the embodiment of the "silicon grade adjustment treatment st1" will be described separately later.

[0033] The desulfurizing agent is preferably formed to have a particle size that can increase the frequency of contact with the crude ferronickel molten liquid. The desulfurizing reaction occurs through a contact reaction between the crude ferronickel molten liquid and the desulfurizing agent at the reaction interface between them. Therefore, it can be said that the smaller the particle size of the desulfurizing agent, the higher the frequency of contact with the crude ferronickel molten liquid. However, if the particle size is too small, the desulfurizing agent may aggregate and form agglomerates in the crude ferronickel molten liquid, and the contact interface with the crude ferronickel molten liquid may be limited to the surface of the agglomerates, reducing the frequency of contact. Furthermore, the smaller the particle size of the desulfurizing agent, the more likely it is that the desulfurizing agent will scatter into the exhaust gas and be carried over in its original form without contributing to the desulfurization reaction. Therefore, the particle size is preferably formed to be within a range of 0.2 mm to 1.2 mm. This can avoid the above-mentioned concerns and effectively increase the desulfurization efficiency.

[0034] The desulfurization agent is preferably added to the ladle at a rate of 4.5 kg / min to 18 kg / min. By setting the lower limit of the addition rate at this value, it is possible to prevent the desulfurization time from becoming too long, and to prevent a drop in the molten metal temperature and the agent from settling in the runner or the like during the casting process.

[0035] Furthermore, the desulfurizing agent can be charged from a charging port of a charging pipe positioned above the crude ferronickel molten metal. Although one charging port may be provided, it is preferable to provide two or more charging ports and charge a predetermined amount of desulfurizing agent from each of the charging ports. The above-mentioned charging rate of the desulfurizing agent (18 kg / min or less) is the rate per charging port. For example, if the desulfurizing agent is charged from only one charging port positioned above the crude ferronickel molten metal at a charging rate of 18 kg / min or less, a long desulfurization time may be required, which may result in a decrease in the temperature of the molten metal or the agent settling in a trough or the like in the subsequent casting process. On the other hand, if a large amount of desulfurizing agent is charged from one port, the desulfurizing agent is likely to aggregate and form lumps in the crude ferronickel molten metal. In this regard, by providing two or more inlets for feeding the predetermined amount of desulfurization agent required for desulfurization treatment and feeding an equal amount of desulfurization agent from each inlet at a feeding rate of 18 kg / min or less, it is possible to prevent the desulfurization time from being long and to perform desulfurization treatment more efficiently and effectively.

[0036] [Silicon quality adjustment processing] The silicon grade adjustment treatment st1 is a treatment for adjusting the silicon (Si) content in the crude ferronickel molten metal so that the "Si / CaO ratio in the crude ferronickel molten metal" of the crude ferronickel charged to the desulfurization step S2 is maintained at a predetermined control value or higher. In this specification, the "Si / CaO ratio in the crude ferronickel molten metal" is the ratio of the silicon (Si) content to the calcium oxide (CaO) content in the crude ferronickel molten metal charged to the desulfurization step S2. Furthermore, the "Si / CaO ratio in the crude ferronickel molten metal" is essentially the ratio between the silicon content in the crude ferronickel molten metal and the amount of calcium oxide (CaO) charged as a desulfurization agent. Therefore, by setting the amount of calcium oxide (CaO) charged to a given value and appropriately adjusting the silicon content, the "Si / CaO ratio in the crude ferronickel molten metal" can be maintained at an optimal value.

[0037] As described above, oxygen is generated in the desulfurization reaction using calcium oxide (Equation 1), but if silicon is present in the crude ferronickel molten metal, the oxygen generated by desulfurization is used in the oxidation reaction of silicon (hereinafter also referred to as the "desiliconization reaction"). Here, the molecular weight of silicon (Si) is 28 [g / mol], and the "desiliconization reaction" is an exothermic reaction as shown in the following Equation 4. (Formula 4):1 / 2O2+1 / 2Si=1 / 2SiO2+405[kJ / mol] In other words, when 1 g of silicon reacts with oxygen generated by the desulfurization reaction, a heat quantity of 28.9 [kJ / g] is provided to the crude ferronickel molten metal, as shown in the following (Equation 5). (Formula 5):810[kJ / mol] / 28[g / mol]=28.9[kJ / g]

[0038] In desulfurization using calcium oxide, a drop in the temperature of the molten metal can occur due to the amount of heat taken from the molten metal depending on the amount of calcium oxide added. However, in the "method for producing ferronickel" of the present invention, by making the "silicon grade adjustment treatment st1" an essential treatment, the silicon content in the crude ferronickel molten metal can be adjusted to a predetermined amount or more, thereby maintaining the "Si / CaO ratio in the crude ferronickel molten metal" at a predetermined value or more, and preventing a drop in the temperature of the crude ferronickel molten metal.

[0039] Regarding the "Si / CaO ratio in the crude ferronickel molten metal," as an example, when 1 g of calcium oxide is added, based on the relationship between the amount of heat absorbed by the desulfurization reaction and the amount of heat generated by the desiliconization reaction, if the silicon content in the crude ferronickel molten metal tapped into the ladle is adjusted to 0.16 g or more (the Si / CaO ratio in the crude ferronickel molten metal is 0.16 or more in mass ratio), as shown in the following (Equation 6), it is possible to avoid a temperature drop caused by the desulfurization reaction. (Formula 6): 1[g]×4.71[kJ / g] / 28.9[kJ / g]=0.16[g]

[0040] Therefore, for example, when the desulfurization step S2 of the "method for producing ferronickel" of the present invention is carried out using 800 kg of calcium oxide as a desulfurization agent, the silicon content can be adjusted so that the "Si / CaO ratio in the molten crude ferronickel" is 0.16 or more in mass ratio, that is, so that the silicon content in the molten crude ferronickel is 130 kg or more. This makes it possible to avoid a decrease in the temperature of the molten crude ferronickel.

[0041] To explain the above treatment more specifically, by adjusting the silicon content in the crude ferronickel molten liquid to a mass ratio of 0.16, it is possible to provide the crude ferronickel molten liquid with an exothermic reaction based on (Equation 4) with an amount of heat that is approximately the same as the amount of heat removed in the endothermic reaction based on (Equation 1).

[0042] Furthermore, by adjusting the silicon content in the crude ferronickel molten metal to be greater than 0.16 in mass ratio, it is possible to provide the crude ferronickel molten metal with an amount of heat greater than the amount of heat taken away in the endothermic reaction based on (Formula 1) through the exothermic reaction based on (Formula 4).

[0043] In this way, by appropriately performing the silicon grade adjustment treatment st1, it is possible to cause a desiliconization reaction sufficient to compensate for the heat lost during the desulfurization reaction, thereby efficiently and reliably suppressing a decrease in the temperature of the molten metal.

[0044] It should be noted that the specific means for carrying out the silicon grade adjustment treatment st1, i.e., the means for adjusting the silicon (Si) content in the crude ferronickel molten metal so that the "Si / CaO ratio in the crude ferronickel molten metal" is maintained at or above a predetermined control value, is not limited to a specific technical means. For example, fluorescent X-ray analysis is performed on the crude ferronickel molten metal after the reduction reaction in the reducing melting furnace or on a test piece obtained from the crude ferronickel molten metal tapped from the reducing melting furnace, and if the silicon grade is below a predetermined value based on the obtained silicon grade, the silicon content may be adjusted, for example, by a method of adding additional silicon-containing auxiliary materials such as ferrosilicon into a ladle.

[0045] Furthermore, when the reducing melting furnace used in the reducing melting treatment is an electric furnace, the silicon content in the crude ferronickel molten metal can also be adjusted by the method described below, thereby making it possible to enjoy the effects of the present invention in the desulfurization treatment without the need to add auxiliary materials such as the above-mentioned ferrosilicon.

[0046] (Method for adjusting silicon content when the reduction melting furnace is an electric furnace) 2 is a schematic diagram showing the structure of an example of an electric furnace (a three-phase AC electrode type circular electric furnace) that is a melting furnace suitable for carrying out the reduction melting step S1 of the "ferronickel manufacturing method" of the present invention. This electric furnace 10 is composed of a furnace body 1 and three rod-shaped electrodes 2 that are installed upright and can move up and down (the number of electrodes is not limited to three and can be any multiple number). When carrying out the reduction melting step S1, inside this electric furnace 10 there are present a metal layer 7 made of molten metal (crude ferronickel molten liquid) and a slag layer 6 made of molten slag, and the surface of the slag layer 6 is covered with ore (calcined ore 5). The calcined ore 5 is supplied from a calcined ore chute (not shown) to above the slag layer 6 inside the furnace.

[0047] Furthermore, in this electric furnace 10, three-phase AC power is supplied to three electrodes 2 inserted and suspended into the furnace through openings in the furnace lid of the furnace body 1, and an arc is generated from this three-phase AC power. The heat of the arc directly melts the cinder 5 (low-current, high-voltage operation method). Alternatively, the three electrodes 2 are immersed down to the slag layer 6, and current is passed directly from the electrodes 2 to the metal and slag, causing resistance heating to melt the cinder 5 indirectly via the slag (high-current, low-voltage operation method). The temperatures of the slag layer 6 and the metal layer 7 are raised to predetermined temperatures, whereby the cinder 5 is reduced and melted to produce metal and slag. The produced metal and slag are separated into the metal layer 7 and the slag layer 6 due to the difference in specific gravity. The metal (crude ferronickel molten metal) is withdrawn through the metal withdrawal port 3, and the slag is withdrawn through the slag withdrawal port 4.

[0048] Typically, a temperature gradient exists between the slag layer 6 and the metal layer 7 inside the electric furnace 10. Therefore, inside the electric furnace 10, a crust layer 8 made of semi-molten material in a non-equilibrium state is formed at the interface between the slag layer 6 and the metal layer 7, as shown in Figure 2. In this specification, this layer made of semi-molten material formed between the metal layer and the slag layer inside the electric furnace is referred to as the "crust layer."

[0049] Now, in an electric furnace having such a structure, when the metal layer 7 and the slag layer 6 come into direct contact inside the electric furnace 10, the silicon component thermodynamically distributes to the slag layer 6, and the silicon content in the metal decreases. For this reason, by appropriately performing operations (operations i) to iii) described below) for maintaining the crust layer 8 formed at the interface between the metal layer 7 and the slag layer 6, it is possible to suppress the outflow of the silicon component to the slag layer 6. In other words, by appropriately using and performing the operations i) to iii), it is possible to adjust the silicon content in the crude ferronickel molten metal. The operations i) to iii) are preferably carried out when the crust layer 8 is tending to shrink in the electric furnace 10. The fact that the crust layer 8 is tending to shrink can be directly inferred, for example, by measuring the furnace level, or can be appropriately carried out by carrying out a step of measuring the silicon grade in the molten ferronickel obtained by the reduction treatment and feeding back the result as a criterion for determining the necessity of the above operations.

[0050] Operation i) is typically performed to adjust the furnace resistance as described above by raising the vertical position of the lower end of the electrode 2, which is installed so that it can be moved up and down, away from the interface between the slag layer 6 and the metal layer 7. This operation prevents the melting of the crust layer 8 due to the proximity of the lower end of the electrode 2, which is the heat source. However, if the lower end of the electrode 2 is positioned too high, there is a risk of dielectric breakdown in the insulation of the electric furnace 10, resulting in a short circuit. Therefore, for a typical three-phase AC circular electric furnace, operation i) is preferably performed within a range that maintains the furnace resistance below 25 mΩ. In the electric furnace 10 shown in Figure 1, the power consumption is expressed by the following equation (6). The electrical conductivity of the accumulated cinder 5 is negligibly low compared to the slag layer 6 and the metal layer 7. Therefore, most of the power required for the melting and reduction reaction, as expressed in equation (7), flows between the electrodes 2 via the slag layer 6 and the metal layer 7. (Equation 7): Power (MW) = Voltage (V) × Current (kA) × √3 = Current (kA) 2 × Furnace resistance (mΩ) × 3 / 1000 The power value in this equation (7) is the target set value, and the furnace resistance can be controlled by changing the voltage value as the set value. Specifically, in an electric furnace, the furnace resistance is controlled by adjusting the position of electrode 2 up or down. For example, to lower the furnace resistance, electrode 2 is lowered.

[0051] Operation ii) is an operation for raising the melting point of the slag forming the slag layer 6. If the melting point of the slag falls below a predetermined temperature, the melting point of the crust layer 8 itself also falls, making the crust layer 8 more susceptible to melting. To prevent this, operation ii) is preferably performed to maintain the melting point of the slag above a predetermined temperature, specifically, above 1520°C. Regarding the melting point of the slag, in actual operation, sampling is performed each time the slag is discharged, and the composition is monitored by X-ray fluorescence analysis. The amount of silicon source and reducing agent added is adjusted accordingly to maintain the melting point above the desired temperature. However, if the slag melting point is too high, it may cause problems with slag discharge from the electric furnace. Therefore, operation ii) is preferably performed within a range that maintains the slag temperature below 1650°C.

[0052] Operation iii) is an operation for increasing the thickness of the slag layer 6. If the thickness of the slag layer 6 is too thin, the distance between the lower end of the electrode 2 and the crust layer 8 becomes small, making the crust layer 8 more likely to melt. To prevent this, it is preferable to maintain the slag thickness at a predetermined thickness or more, specifically, 50 cm or more, by operation iii). The thickness of the slag layer 6 can be estimated by inserting a measuring rod into the electric furnace 10 and then removing it, and then measuring the deposits on the measuring rod. The thickness of the slag layer 6 can be controlled by adjusting the amount of slag discharged from the furnace body 1 based on the measurement results. However, if the slag layer 6 is too thick, there is an increased risk of refractory melting in the upper part of the furnace body 1. Therefore, as a general guideline, operation iii) is preferably performed within a range that maintains the thickness of the slag layer 6 at 100 cm or less. [Example]

[0053] The present invention will be explained in more detail below by showing examples of test operations, but the present invention is not limited to the following examples in any way.

[0054] [Example] (reduction melting process) Nickel oxide ore "calcined ore" was charged into a three-phase AC electrode-type circular electric furnace to carry out a reduction melting step S1. In addition, in the reduction melting step S1, the following operation (operation i)) was also carried out as a silicon quality adjustment treatment st1. (Silicon quality adjustment treatment) An operation (operation i) was performed in which the electrode was raised to move it away from the interface between the slag layer and the metal layer. This resulted in a furnace resistance of 16 mΩ. The amount of silica added was also adjusted, raising the MgO / SiO2 content of the slag by 0.04, and raising the melting point of the slag to 1520-1650°C. The thickness of the slag layer was also increased to 60 cm. These operations maintained the silicon content of the crude ferronickel molten metal tapped into the ladle at 130 kg or more. (Desulfurization process) 30 t of the crude ferronickel molten metal obtained in the reduction melting step S1 was received in a ladle, and while the crude ferronickel molten metal was being stirred with a stirring device equipped with a stirring blade, a desulfurization agent was added from an inlet at the top of the molten metal to perform the desulfurization step S2. 800 kg of calcium oxide (CaO) with a particle size range of 0.2 mm to 1.2 mm was used as the desulfurization agent. The desulfurization agent was added from two desulfurization agent inlets provided symmetrically with respect to the stirring blade located at the center of the ladle. The desulfurization agent was added at a rate of 18 kg / min per inlet. It was confirmed by fluorescent X-ray analysis that the Si / CaO ratio in the crude ferronickel molten metal after the addition of the desulfurization agent was 0.163. The temperature of the molten metal was measured before and after the desulfurization step S2 to determine the temperature decrease of the crude ferronickel molten metal due to the desulfurization treatment.

[0055] [Reference example] In the desulfurization step S2, a test operation was carried out under the same conditions as in the example, except that calcium carbide (CaC2) was used as the desulfurization agent instead of calcium oxide (CaO).

[0056] The temperature change of the crude ferronickel molten metal in the desulfurization process in each of the test operations of the Example and the Reference Example is shown in Table 1 below.

[0057] [Table 1]

[0058] In the Reference Example, calcium carbide is used as the desulfurization agent, and the desulfurization reaction is an exothermic reaction. Despite this, the temperature change in the Example is approximately the same as that in the Reference Example. From this result, it can be seen that by adjusting the Si / CaO ratio in the crude ferronickel molten metal to be greater than 0.16, the amount of heat lost in the desulfurization reaction can be compensated for by the desiliconization reaction.

[0059] From a comparison of the temperature changes in the above-mentioned Examples and Reference Examples, it can be seen that the "method for producing ferronickel" of the present invention is a production method that can appropriately maintain the quality of ferronickel by suppressing a decrease in the temperature of the molten crude ferronickel, even when calcium oxide (CaO) is used as a desulfurizing agent in smelting ferronickel, thereby avoiding the risk of a decrease in the quality of ferronickel caused by the temperature decrease (risk of a decrease in the reactivity of the molten ferronickel, risk of casting becoming difficult when a casting step is performed in which ferronickel is processed into shot). [Explanation of symbols]

[0060] 1 Furnace body 2 electrodes 3 Metal outlet 4 Slag extraction port 5. Burning Ore 6 Slag layer 7 Metal Layer 8 Crust layer 10 Electric furnace S1 Reduction melting process S2 Desulfurization process st1 Silicon grade adjustment treatment

Claims

1. A method for producing ferronickel, in which ferronickel containing nickel as a main component is obtained from nickel oxide ore containing nickel, iron, silicon, and sulfur, comprising the steps of: a reduction-smelting step of reducing the nickel oxide ore in a melting furnace to obtain a crude ferronickel molten metal having a nickel content of 14% by mass or more and a silicon content of 3% or less; a desulfurization step of adding a desulfurization agent to the crude ferronickel molten metal to remove sulfur; comprising In the desulfurization step, calcium oxide is used as the desulfurization agent, A method for producing ferronickel, comprising the steps of: adjusting the silicon content in the crude ferronickel molten metal so that the Si / CaO ratio in the crude ferronickel molten metal is maintained at or above a predetermined control value; and performing a silicon quality adjustment treatment prior to the desulfurization step.

2. the predetermined control value of the Si / CaO ratio in the crude ferronickel molten metal is set to 0.16 or more in mass ratio; The method for producing ferronickel according to claim 1.

3. The melting furnace for carrying out the reduction melting step is an electric furnace in which a plurality of rod-shaped electrodes are vertically movable, and the silicon grade adjustment process is carried out by performing any one or more of the following operations i) to iii): The method for producing ferronickel according to claim 1 or 2. i) Raise the electrode away from the interface between the slag and metal layers. ii) Increase the melting point of the slag that forms the slag layer. iii) Increasing the thickness of the slag layer.

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