Patent Description:
As a method for smelting nickel oxide ore which is one kind of oxide ore and called limonite or saprolite, a dry smelting method in which nickel mat is produced by using an smelting furnace, a dry smelting method in which ferronickel, which is an alloy of iron and nickel, is produced by using a rotary kiln or a movable hearth furnace, a hydrometallurgical method in which a mixed sulfide is produced by using an autoclave, and the like are known.

A treatment for forming nickel oxide ore of a raw material into a lump product by crushing the nickel oxide ore into a proper size and the like is performed as a pretreatment in order to advance the reaction particularly in a case in which nickel oxide ore is reduced and smelted by a dry smelting method among the various methods described above.

Specifically, when nickel oxide ore is formed into a lump product, that is, a lump is formed from a powdery or granular ore, it is general that the nickel oxide ore is mixed with other components, for example, a binder, a reducing agent such as coke to prepare a mixture and the mixture is further subjected to moisture adjustment and the like, then charged into a lump product manufacturing machine, and formed into a lump product (indicating a pellet, a briquette, or the like. Hereinafter simply referred to as the "pellet") having, for example, one side or a diameter of about <NUM> to <NUM>.

The pellet obtained as a lump product is required to exhibit gas permeability to a certain extent in order to "emit" the moisture contained. Furthermore, the composition of the reduction product to be obtained is ununiform and a trouble that the metal is dispersed or unevenly distributed is caused when the reduction does not uniformly proceed in the pellet in the subsequent reduction treatment. For this reason, it is important to uniformly mix the mixture when fabricating pellets or to maintain the temperature as constant as possible when reducing the pellets obtained.

In addition, it is also a significantly important technique to coarsen the metal (ferronickel) to be generated by the reduction treatment. It is difficult to separate the ferronickel from the slag to be generated at the same time, and the recovery rate (yield) as ferronickel greatly decreases in a case in which the ferronickel generated has a fine size of, for example, several tens of micrometers to several hundreds of micrometers or less. For this reason, a treatment for coarsening ferronickel after being reduced is required.

Furthermore, it is also an important technical problem how the smelting cost can be suppressed low, and a continuous treatment that can be operated in a compact facility is desired.

For example, Patent Document <NUM> discloses a technique intended to further enhance the productivity of granular metal when producing a granular metal by heating an agglomerated product containing a metal oxide and a carbonaceous reducing agent and thus reducing and melting the metal oxide contained in the agglomerated product. Specifically, a method for producing a granular metal is disclosed in which an agglomerated product containing a metal oxide and a carbonaceous reducing agent is supplied onto the hearth of a moving bed type reduction melting furnace and heated to reduce and melt the metal oxide and the granular metal obtained is cooled, then discharged to the outside of the furnace, and recovered. Moreover, this technique is characterized in that an agglomerated product having an average diameter of <NUM> or more and <NUM> or less is supplied onto the hearth when performing heating by setting the base density of the agglomerated product on the hearth to <NUM> or more and <NUM> or less where the base density denotes the relative value of the projected area ratio of the agglomerated product spread on the hearth onto the hearth with respect to the largest projected area ratio of the agglomerated product onto the hearth when the distance between the agglomerated products spread on the hearth is taken as <NUM> as well as the furnace temperature in the first half region in which the iron oxide in the agglomerated product is solid-reduced of the furnace is set to <NUM> to <NUM> and the furnace temperature in the second half region in which the reduced iron in the agglomerated product is carburized, melted and aggregated of the furnace is set to <NUM> to <NUM>, and according to such a method, it is said that the productivity of granular metal iron can be improved as the base density and average diameter of the agglomerated product are controlled concurrently. <CIT> discloses a nickel oxide reduction method.

Indeed, it is also considered that the productivity of granular metal iron can be improved as the base density and average diameter of the agglomerated product are controlled as compared with the technique known before the technique disclosed in Patent Document <NUM> described above is proposed. However, this technique is merely a technique concerning the reactions which take place outside the agglomerated product, and the most important factor in the reduction reaction is the internal state of the agglomerate in which the reduction reaction takes place.

In other words, it can be said that, for example, it is possible to further enhance the reaction efficiency, also to uniformly conduct the reduction reaction, and to produce a high quality metal by controlling the reduction reaction in the agglomerate.

In addition, the yield at the time of production of the agglomerate decreases and this leads to an increase in cost as the diameter of the agglomerate is set to be in a regulated range as in the technique disclosed in Patent Document <NUM>. Furthermore, the agglomerate cannot be laminated unless otherwise close-packed when the base density of the agglomerate is set to be in the range of <NUM> or more and <NUM> or less, and a significantly inefficient treatment step is performed and this leads to an increase in manufacturing cost.

Furthermore, there is a significant problem in terms of operation cost as well in the process using the so-called total melting method in which all the raw materials are melted and reduced as the technique disclosed in Patent Document <NUM>. For example, a high temperature of <NUM> or more is required in order to completely melt nickel oxide ore of a raw material, but large energy cost is required in order to achieve such a high temperature condition and repair cost is also required since the furnace to be used at such a high temperature is likely to be damaged. In addition, it is extremely inefficient to completely melt nickel oxide ore of a raw material since the nickel oxide ore contains nickel at only about <NUM>% and all the components including components which are contained in a great amount but not required to be recovered are melted even though components other than iron corresponding to nickel are not required to be recovered.

For this reason, reduction methods by partial melting have been investigated in which only nickel required is preferentially reduced and iron contained in a much greater amount than nickel is only partially reduced. However, in such a partial reduction method (or also referred to as a nickel preferential reduction method), the reduction reaction is conducted while maintaining the raw materials in a semi-solid state in which the raw materials are not completely melted and thus it is not easy to control the reaction so that iron is only partially reduced while nickel is <NUM>% completely reduced. For this reason, there is a problem that partial variations in the reduction in the raw material occur, the recovery rate of nickel decreases, and it is thus difficult to perform an efficient operation.

As described above, there have been a number of problems in order to produce a high quality metal while diminishing the manufacturing cost as well as to improve the productivity when mixing nickel oxide ore of a raw material, reducing the mixture, and thus producing a metal.

The present invention has been proposed in view of such circumstances, and an object thereof is to provide a method by which a high quality metal can be inexpensively and efficiently produced as well as the recovery rate of metal is enhanced and thus the productivity is improved in a smelting method for producing a metal by reducing a mixture containing an oxide ore such as nickel oxide ore.

The inventors of the present invention have conducted intensive investigations to solve the above-mentioned problems. As a result, it has been found out that a high quality metal having a high nickel grade can be efficiently produced by depositing a specific compound on the surface of a mixture obtained by mixing an oxide ore of a raw material with a reducing agent and subjecting the mixture to a reduction treatment in that state, whereby the present invention has been completed.

The invention is laid out in the appended claims.

According to the present invention, it is possible to inexpensively and efficiently produce a high quality metal as well as to enhance the recovery rate of metal and thus to improve the productivity in a smelting method for producing a metal by reducing a mixture containing an oxide ore such as nickel oxide ore.

Hereinafter, specific embodiments of the present invention (hereinafter referred to as the "present embodiments") will be described in detail. In addition, in the present specification, the notation "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

The oxide ore smelting method according to the present invention is a method for producing a metal, which is a reduction product, by using an oxide ore as a raw material, mixing the oxide ore with a carbonaceous reducing agent to obtain a mixture and subjecting the mixture obtained to a reduction treatment at a high temperature. Examples thereof may include a method for producing ferronickel, which is an alloy of iron and nickel, by using nickel oxide ore containing nickel oxide, iron oxide and the like of an oxide ore as a raw material, mixing the nickel oxide ore with a carbonaceous reducing agent, preferentially reducing nickel contained in the mixture at a high temperature, and partially reducing iron.

Specifically, the oxide ore smelting method according to the present invention is characterized in that the reduction treatment is performed in a state in which one or more kinds of compounds (hereinafter also referred to as the "surface deposits") from a carbonaceous reducing agent is deposited on the surface of the mixture in a method for obtaining a metal, which is a reduction product, and slag by mixing an oxide ore with a carbonaceous reducing agent, heating the mixture obtained, and subjecting the mixture to a reduction treatment.

According to such a smelting method, it is possible to enhance the metallized rate of nickel or the like and to produce a high quality metal having a high grade of metal such as nickel by performing a reduction treatment in a state in which a surface deposit is deposited on the surface of a mixture containing an oxide ore and a carbonaceous reducing agent. In addition, it is possible to inexpensively and efficiently perform the treatment since the method is an extremely simple method in which a surface deposit is deposited on the surface of a mixture obtained by mixing at least an oxide ore with a carbonaceous reducing agent.

Hereinafter, as a specific embodiment of the present invention (hereinafter referred to as the "present embodiment"), a method for smelting nickel oxide ore will be described as an example. As described above, the nickel oxide ore, which is a raw material for smelting, contains at least nickel oxide (NiO) and iron oxide (Fe<NUM>O<NUM>) and an iron-nickel alloy (ferronickel) can be produced as a metal by performing a reduction treatment using the nickel oxide ore as a raw material for smelting.

The method for smelting nickel oxide ore according to the present embodiment is a method for generating ferronickel, which is a metal, as a reduction product and slag by mixing nickel oxide ore with a carbonaceous reducing agent to obtain a mixture and subjecting the mixture to a reduction treatment. Incidentally, ferronickel, which is a metal, can be recovered from a mixture which contains metal and slag and is obtained through a reduction treatment by separating the metal.

<FIG> is a flow chart illustrating an example of the flow of a method for smelting nickel oxide ore. As illustrated in <FIG>, this smelting method includes a mixing treatment step S1 for mixing raw materials including nickel oxide ore, a mixture-molding step S2 for molding the mixture obtained into a predetermined shape, a reducing step S3 for reducing and heating the mixture (pellet) molded at a predetermined reducing temperature, and a separating step S4 for separating the metal and slag generated in the reducing step S3 from each other and recovering the metal.

The mixing treatment step S1 is a step for mixing raw material powders including nickel oxide ore to obtain a mixture. Specifically, in the mixing treatment step S1, a carbonaceous reducing agent is added to and mixed with nickel oxide ore, which is a raw material ore, and powders of iron ore, a flux component, a binder and the like having a particle diameter of, for example, about from <NUM> to <NUM> as additives of arbitrary components are added to and mixed with the mixture, thereby obtaining a mixture. Incidentally, the mixing treatment can be performed by using a mixing machine or the like.

The nickel oxide ore, which is a raw material ore, is not particularly limited, but limonite ore, saprolite ore and the like can be used. Incidentally, the nickel oxide ore contains at least nickel oxide (NiO) and iron oxide (Fe<NUM>O<NUM>).

The carbonaceous reducing agent is a coal and/or a coke powder. Incidentally, it is preferable that this carbonaceous reducing agent has a size equivalent to the particle size and particle size distribution of the nickel oxide ore, which is a raw material ore, since these materials are likely to be uniformly mixed and the reduction reaction is also likely to uniformly proceed.

The amount of the carbonaceous reducing agent mixed is adjusted so that the proportion of carbon amount is <NUM>% by mass or more and <NUM>% by mass or less and more preferably <NUM>% by mass or more and <NUM>% by mass or less when the total value (also conveniently referred to as the "total value of chemical equivalents") of a chemical equivalent required for reducing the entire amount of nickel oxide constituting the nickel oxide ore into nickel metal and a chemical equivalent required for reducing iron oxide (ferric oxide) into iron metal is taken as <NUM>% by mass. The reduction of nickel can be efficiently advanced and the productivity is improved by setting the amount of the carbonaceous reducing agent mixed to a proportion to be <NUM>% by mass or more with respect to <NUM>% by mass of the total value of chemical equivalents in this manner. On the other hand, it is possible to suppress the amount of iron reduced, to prevent a decrease in nickel grade, and to produce high quality ferronickel by setting the proportion to <NUM>% by mass or less with respect to <NUM>% by mass of the total value of chemical equivalents. The amount of the carbonaceous reducing agent mixed is set to a proportion of carbon amount to be <NUM>% by mass or more and <NUM>% by mass or less with respect to <NUM>% by mass of the total value of chemical equivalents in this manner since it is possible to uniformly generate a shell (metal shell) generated from a metal component on the surface of the mixture, to improve the productivity, and also to obtain high quality ferronickel having a high nickel grade.

In addition, in a case in which at least a carbonaceous reducing agent is deposited on the surface of a mixture obtained by mixing nickel oxide ore with a carbonaceous reducing agent as a surface deposit and the reduction treatment is performed in this state in the reducing step S3 of the subsequent step, it is preferable that the amount of the carbonaceous reducing agent present inside the mixture is set to a proportion of <NUM>% by mass or less when the total value of chemical equivalents described above is taken as <NUM>% by mass. Incidentally, the carbonaceous reducing agent to be deposited on the surface of the mixture to be subjected to the reduction treatment is also referred to as the "carbonaceous reducing agent for surface deposition" for convenience in order to distinguish this carbonaceous reducing agent from the carbonaceous reducing agent which constitutes the mixture together with the nickel oxide ore and is present inside the mixture.

In a case in which the carbonaceous reducing agent for surface deposition is deposited on the surface of the mixture and the reduction treatment is performed in this manner, it is possible to uniformly form a metal shell on the surface of the mixture (pellet) by the reduction treatment as the amount (mixed amount) of the carbonaceous reducing agent to be contained in the mixture is adjusted so as to be a proportion to be <NUM>% by mass or less with respect to <NUM>% by mass of the total value of chemical equivalents although it will be described in detail later. In addition, it is possible to suppress an increase in the amount of metal iron due to excessive metalation of iron by the reduction reaction and to prevent a decrease in the nickel grade in ferronickel.

In addition, as the iron ore, which is an additive of an arbitrary component, for example, iron ore having an iron grade of about <NUM>% or more, hematite to be obtained by hydrometallurgy of nickel oxide ore, and the like can be used.

In addition, examples of the flux component may include calcium oxide, calcium hydroxide, calcium carbonate, and silicon dioxide. In addition, examples of the binder may include bentonite, a polysaccharide, a resin, water glass, and dehydrated cake.

In the mixing treatment step S1, a mixture is obtained by uniformly mixing raw material powders including nickel oxide ore as described above. Upon this mixing, kneading may be performed at the same time as mixing or after mixing in order to enhance the mixing property. Specifically, kneading can be performed by using, for example, a twin-screw kneader and the like, and it is possible to improve adhesive property of the respective particles and to decrease voids as well as to uniformly mix the materials by applying a shear force to the mixture and untangling the aggregation of the carbonaceous reducing agent, raw material powders and the like by kneading the mixture. It is possible to uniformly conduct the reaction and to shorten the reaction time of the reduction reaction as well as the reduction reaction is likely to take place by this. In addition, it is possible to diminish variations in the quality. Moreover, it is possible to perform a highly productive treatment and to produce high quality ferronickel as a result.

In addition, after kneading, the mixture may be extruded by using an extruding machine. It is possible to obtain a still higher kneading effect by extruding the mixture by using an extruding machine in this manner.

Incidentally, an example of the composition (% by weight) of a part of raw material powders to be mixed in the mixing treatment step S1 are presented in the following Table <NUM>, but the composition of the raw material powders is not limited thereto.

The mixture-molding step S2 is a step for molding the mixture obtained in the mixing treatment step S1. Specifically, the mixture obtained by mixing the raw material powders is molded into a lump (lumped product, hereinafter also referred to as the "pellet") having a certain size or larger. Hence, the mixture-molding step S2 can also be said to be a pellet producing step.

The molding method is not particularly limited, but moisture is added to the mixture in an amount required for forming the mixture into a lump product and the mixture is molded into a pellet having a predetermined shape by using, for example, a lump product manufacturing apparatus (a tumbling granulator, a compression molding machine, an extrusion molding machine, or the like, or also referred to as a pelletizer).

The shape of the lumped product (pellet) to be obtained by molding the mixture can be, for example, a rectangular parallelepiped shape, a cylindrical shape, or a spherical shape. It is possible to easily mold the mixture and to diminish the cost required for molding by adopting such a shape. In addition, it is possible to suppress the generation of defective products and to make the quality of the pellets to be obtained uniform since the shape described above is a simple shape but is not complicated.

In addition, as the shape of the lumped product, it is preferable that the pellets can be treated in a state of being laminated in the treatment in the reducing step of the next step, and it is easy to place the pellets in the reducing furnace by laminating and to increase the throughput to be subjected to the reduction treatment when the pellet has a rectangular parallelepiped shape, a cylindrical shape, a spherical shape or the like in this regard as well. In addition, it is possible to increase the throughput at the time of reduction without enlarging one pellet by subjecting the pellets to the reduction treatment by laminating in this manner, and thus it is easy to handle the pellets, the pellets does not collapse and the like at the time of moving and the like, and the generation of defects and the like can be suppressed.

The volume of the mixture (pellet) molded is not particularly limited, but it is preferably <NUM><NUM> or more. The molding cost increases and it takes time and labor to charge the pellets into the reducing furnace when the volume of the pellets is too small. In addition, the proportion of the surface area with respect to the entire pellets increases when the volume of the pellets is small, and thus a difference in the degree of reduction between the surface and inside of the pellet is likely to occur, there is a possibility that it is difficult to uniformly advance the reduction, and it is difficult to produce high quality ferronickel. On the other hand, it is possible to effectively diminish the molding cost and it is easy to handle the pellets when the volume of the pellets composed of the mixture is <NUM><NUM> or more. In addition, it is possible to stably obtain high quality ferronickel.

After the mixture is molded, the mixture may be subjected to a drying treatment. There is a case in which a certain amount of moisture may be contained in the mixture, and there is concern that the mixture is broken into fragments when the internal moisture evaporates and expands at a time by a sharp increase in the temperature at the time of the reduction treatment. It is possible to provide a step for subjecting the mixture molded to a drying treatment from the viewpoint of preventing such expansion.

Specifically, in the drying treatment, for example, a treatment can be performed so that the pellet has a solid content of about <NUM>% by weight and a water content of about <NUM>% by weight. For example, the pellets are dried by blowing hot air at from <NUM> to <NUM> thereto.

Incidentally, fissures and breaks may be present on the mixture before and after being subjected to the drying treatment in a case in which the mixture is a relatively large pellet. It is not a significant problem that the surface area increases by breaks and the like since the influence thereof is slight in a case in which the lump is large. For this reason, there is particularly no problem even when breaks and the like are present on the molded pellets to be subjected to the reduction treatment.

An example of the composition (parts by weight) of solid components in the mixture after being subjected to a drying treatment is presented in the following Table <NUM>. Incidentally, the composition of the mixture is not limited to this.

In the reducing step S3, the mixture molded through the mixture-molding step S2 is charged into a reducing furnace and reduced and heated at a predetermined reducing temperature. The smelting reaction (reduction reaction) proceeds and a metal, which is a reduction product, and slag are generated by the reduction and heat treatment in this reducing step S3.

In the reducing step S3, the slag in the mixture melts to form a liquid phase, but the metal and the slag which have been already separately generated by the reduction treatment do not mix with each other but form a mixture in which the metal and the slag are present together as separate phases of a metal solid phase and a slag solid phase by subsequent cooling. The volume of this mixture is contracted to a volume to be about from <NUM>% to <NUM>% of the volume of the mixture charged.

Meanwhile, the present embodiment is characterized in that the treatment is performed in a state in which a carbonaceous reducing agent (carbonaceous reducing agent for surface deposition), is deposited on the surface of the mixture when subjecting the mixture (pellet) to the reduction treatment in the reducing furnace.

Specifically, a layer of a carbonaceous reducing agent is formed on the surface of the mixture in a case in which a carbonaceous reducing agent for surface deposition is used as the surface deposit, and it is possible to effectively form a shell (metal shell) generated from a metal component on the surface by performing the reduction treatment in this state. This makes it possible to prevent the carbonaceous reducing agent (reducing agent component) present inside the mixture from leaking from the mixture and to stably conduct the reduction reaction. In addition, it is possible to suppress the collapse at the time of the reduction and heat treatment since the strength of the mixture subjected to the reduction treatment is maintained. For these reasons, it is possible to efficiently produce high quality ferronickel without causing divergence or variations in the composition.

As the carbonaceous reducing agent for surface deposition, a coal powder, a coke powder and the like can be used in the same manner as the carbonaceous reducing agent present inside the mixture. In addition, the size and shape of the carbonaceous reducing agent for surface deposition are also not particularly limited, and it is preferable to use one having a size of about from several micrometers to several hundreds of micrometers, for example, in a case in which the mixture is a spherical one having a diameter of from several millimeters to several tens of millimeters.

In the case of using a carbonaceous reducing agent for surface deposition, it is preferable that the amount of the carbonaceous reducing agent for surface deposition (the amount deposited on the mixture) is set to a proportion of <NUM>% by mass or more and <NUM>% by mass or less when the total value of chemical equivalents of the carbonaceous reducing agent required for reducing nickel oxide and iron oxide contained in the mixture to be subjected to the reduction treatment without excess or deficiency is taken as <NUM>% by mass. In addition, the amount is more preferably set to a proportion of <NUM>% by mass or more and <NUM>% by mass or less and still more preferably set to a proportion of <NUM>% by mass or more and <NUM>% by mass or less.

There is a possibility that the effect to be obtained by depositing the carbonaceous reducing agent on the surface is not sufficiently obtained and the reaction for generating a metal shell does not efficiently proceed when the amount of the carbonaceous reducing agent for surface deposition deposited is a proportion to be less than <NUM>% by mass with respect to <NUM>% by mass of the total value of chemical equivalents described above. On the other hand, the reduction of iron oxide in the metal shell formed proceeds too much and there is a possibility that the grade of nickel in ferronickel to be obtained decreases when the deposited amount is a proportion to be more than <NUM>% by mass with respect to <NUM>% by mass of the total value of chemical equivalents. In addition, it is disadvantageous in terms of cost that the deposited amount exceeds <NUM>% by mass since the amount of the carbonaceous reducing agent for surface deposition is too excessive.

The method for depositing the carbonaceous reducing agent for surface deposition on the surface of the mixture is not particularly limited, but it is preferable to coat the carbonaceous reducing agent for surface deposition on the surface of the mixture so as to be uniformly present on the surface of the mixture. For example, the carbonaceous reducing agent for surface deposition is deposited and coated on the mixture while rolling the mixture on the carbonaceous reducing agent for surface deposition spread on a flat sheet. Alternatively, the carbonaceous reducing agent for surface deposition may be deposited on the mixture by being sprinkled from above the mixture.

Excess moisture at, for example, about <NUM>% by weight is contained in the pellet and the pellet is in a sticky state when the mixture is lumped into a pellet. Hence, it is possible to effectively deposit the carbonaceous reducing agent for surface deposition on the surface by rolling the mixture (pellet) on the carbonaceous reducing agent for surface deposition or sprinkling the carbonaceous reducing agent for surface deposition from above. The same applies to a case in which a metal oxide is applied to be described later.

A pellet can be produced, charged into a reducing furnace, and subjected to a reduction treatment by depositing a carbonaceous reducing agent for surface deposition on the surface of the mixture in this manner.

In addition, the carbonaceous reducing agent for surface deposition may be deposited so as to be put on a part of the surface of the mixture, particularly on the surface of the upper part, for example, as illustrated in <FIG> to be described in detail later. Furthermore, the carbonaceous reducing agent for surface deposition may be deposited so as to surround the mixture as illustrated in <FIG> to be described later. Incidentally, in this case, the carbonaceous reducing agent for surface deposition may be put on the upper surface of the mixture (pellet) inside the reducing furnace or a lump of the carbonaceous reducing agent for surface deposition may be prepared in the reducing furnace in advance and the mixture may be buried therein.

The reducing furnace to be used for the reduction and heat treatment is a rotary hearth furnace. By using a rotary hearth furnace as a reducing furnace, the reduction reaction continuously proceeds and it is possible to complete the reaction in one facility and to more accurately control the treatment temperature as compared to a case of performing the treatments in the respective steps by using separate furnaces.

In addition, it is possible to decrease loss of heat (heat loss) between the respective treatments and to more efficiently perform the operation by using a rotary hearth furnace. In other words, in the case of performing the reactions by using separate furnaces, the temperature temporarily drops and heat loss occurs as the container, in which the mixture is enclosed, is exposed to the outside air or a state close thereto when being moved from one furnace to another furnace, and a change in the reaction atmosphere is also caused. As a result, the reaction does not start immediately when the container is recharged into the furnace in order to perform the next treatment.

In contrast, by performing the respective treatments in one facility by using a rotary hearth furnace, the furnace atmosphere can be accurately controlled as well as the heat loss diminishes, and it is thus possible to more effectively advance the reaction. These make it possible to more effectively obtain high quality ferronickel having a high nickel grade.

The rotary hearth furnace which has a circular shape and is partitioned into a plurality of treatment regions. In the rotary hearth furnace, each treatment is performed in each region while the furnace rotates in a predetermined direction. In this rotary hearth furnace, the treatment time in each region can be adjusted by controlling the time (moving time, rotating time) when the mixture passes through each region, and the mixture is smelted every time the rotary hearth furnace rotates one time. In addition, the movable hearth furnace may be a roller hearth kiln or the like.

In the reduction treatment using a reducing furnace, so-called partial reduction is performed in which nickel oxide contained in the nickel oxide ore, which is a raw material ore, is preferentially reduced as completely as possible but iron oxide contained in the nickel oxide ore is only partially reduced so as to obtain the intended ferronickel having a high nickel grade.

The reducing temperature is in a range of <NUM> or more and <NUM> or less and more preferably in a range of <NUM> or more and <NUM> or less. By performing the reduction in such a temperature range, it is possible to uniformly conduct the reduction reaction and to generate a metal (ferronickel) having diminished variations in quality. In addition, it is possible to conduct the desired reduction reaction in a relatively short time by performing the reduction at a reducing temperature in a more preferable range of <NUM> or more and <NUM> or less.

Incidentally, in the reduction treatment, the internal temperature of the reducing furnace is raised by using a burner or the like until the reducing temperature reaches the range described above and the temperature after being raised is maintained.

In addition, in the reducing step S3, a carbonaceous reducing agent (hereinafter also referred to as the "hearth carbonaceous reducing agent") may be spread on the hearth of the reducing furnace in advance and the mixture may be placed on the spread hearth carbonaceous reducing agent and subjected to the treatment when charging the mixture into the reducing furnace. In addition, a hearth covering material such as alumina, zirconia, or magnesia may be laid on the hearth and the mixture may also be placed thereon and subjected to the treatment. Incidentally, as the hearth covering material, one containing an oxide as the main component can be used.

It is possible to suppress the direct reaction between the hearth and the mixture, to prevent fusion of the mixture to the hearth, and to extend the life span of the hearth by laying a carbonaceous reducing agent, a hearth covering material and the like on the hearth of the reducing furnace, placing the mixture thereon, and performing the reduction treatment in this manner.

In the separating step S4, the metal and the slag which have been generated in the reducing step S3 are separated from each other and the metal is recovered. Specifically, the metal phase in the mixture (mixed product), which contains a metal phase (metal solid phase) and a slag phase (slag solid phase) and is obtained by the reduction and heat treatment of the mixture, is separated from the slag phase and recovered.

As a method for separating the metal phase and slag phase in the mixed product which is composed of the metal phase and the slag phase and is obtained as a solid from each other, for example, methods such as separation by specific gravity and separation by magnetic force can be utilized in addition to removal of unnecessary substances by sieving.

In addition, the metal phase and slag phase obtained can be easily separated from each other since these exhibit poor wettability, and it is possible to easily separate the metal phase and slag phase in the mixed product from each other by imparting an impact to the large mixed product obtained by the treatment in the reducing step S3 described above, for example, falling down the large mixed product at a predetermined falling distance or applying a predetermined vibration to the large mixed product at the time of sieving.

The metal phase is recovered by separating the metal phase and the slag phase from each other in this manner.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples at all.

A mixture was obtained by mixing nickel oxide ore as a raw material ore, iron ore, quartz sand and limestone which were flux components, a binder, and a carbonaceous reducing agent (coal powder, carbon content: <NUM>% by weight, average particle diameter: about <NUM>). The carbonaceous reducing agent was contained in an amount to be a proportion of from <NUM>% by mass to <NUM>% by mass depending on the sample when the total value of the amounts of the carbonaceous reducing agent required for reducing nickel oxide (NiO) contained in the nickel oxide ore, which was a raw material ore, and iron oxide (Fe<NUM>O<NUM>) without excess or deficiency was taken as <NUM>% by mass.

Next, moisture was appropriately added to the mixture of raw material powders thus obtained and the mixture was kneaded by hand to form a spherical mixture.

Subsequently, a coal powder, which was a carbonaceous reducing agent (carbonaceous reducing agent for surface deposition), was uniformly coated and deposited on the surface of the spherical mixture obtained. The amount of the carbonaceous reducing agent for surface deposition deposited was set to an amount to be a proportion of from <NUM>% by mass to <NUM>% by mass depending on the sample when the amount of the carbonaceous reducing agent for surface deposition required for reducing the nickel oxide and iron oxide contained in the mixture without excess or deficiency was taken as <NUM>% by mass.

Thereafter, the mixture was subjected to a drying treatment in which hot air at from <NUM> to <NUM> was blown onto the mixture so that the mixture had a solid content of about <NUM>% by weight and a water content of about <NUM>% by weight, thereby producing a spherical mixture (pellet, diameter: <NUM>). Incidentally, the composition (excluding carbon) of solid components in the pellets after being subjected to the drying treatment is presented in the following Table <NUM>.

The pellets produced were charged into a reducing furnace and subjected to a reduction treatment. Specifically, "ash" containing SiO<NUM> as the main component and a small amount of oxides such as Al<NUM>O<NUM> and MgO as other components was spread on the hearth of the reducing furnace in advance and the pellets were placed thereon. Incidentally, in the pellets in which the carbonaceous reducing agent (coal powder) was deposited on the surface, the coal powder which was not deposited on the surface of the pellets because of a great amount was deposited on the surface of the pellets again by being sprinkled from above after the pellets were placed on the hearth.

Thereafter, a nitrogen atmosphere which substantially did not contain oxygen was set, and the pellets were charged into the reducing furnace. Incidentally, the temperature condition at the time of charging was set to <NUM> ± <NUM>.

Next, the reducing temperature was set to <NUM>, and the pellets were reduced and heated in the reducing furnace. The treatment time was set to <NUM> minutes so that a metal shell was generated on the surface of the pellet and the reduction in the pellet, which was a mixture, efficiently proceeded. After the reduction treatment, the sample was rapidly cooled to room temperature in the nitrogen atmosphere and then taken out into the air.

For the samples taken out from the reducing furnace after being subjected to the reduction treatment, the metallized rate of nickel and the nickel content rate in the metal were analyzed by using an ICP emission spectroscopic analyzer (SHIMAZU S-<NUM> model) and calculated. Incidentally, the metallized rate of nickel was determined by Equation (<NUM>) and the nickel content rate in the metal was determined by Equation (<NUM>). <MAT> <MAT>.

The amount of coal powder (carbonaceous reducing agent for surface deposition) deposited and the content of coal powder (carbonaceous reducing agent) contained inside the pellet in each pellet sample are presented in the following Table <NUM>. In addition, the measurement results acquired by ICP analysis are presented concurrently.

As presented in the results of Table <NUM>, it has been found that it is possible to favorably metallize nickel in the pellet and to produce high grade ferronickel having a nickel content rate of from <NUM>% to <NUM>% as a pellet in which a carbonaceous reducing agent for surface deposition is deposited on the surface is subjected to a reduction treatment (Example <NUM> to Example <NUM>).

Claim 1:
An oxide ore smelting method comprising: mixing an oxide ore and a carbonaceous reducing agent; molding the resulting mixture into a solid pellet; heating the pellet in a rotary hearth furnace; and subjecting the pellet to a reduction treatment to obtain a metal of a reduction product and slag, wherein
the oxide ore is nickel oxide ore,
the carbonaceous reducing agent is coal and/or coke,
an amount of the carbonaceous reducing agent present inside the mixture together with the nickel oxide ore is set to a proportion of <NUM>% by mass or more and <NUM>% by mass or less when an amount of the carbonaceous reducing agent required for reducing nickel oxide and iron oxide contained in the mixture without excess or deficiency is taken as <NUM>% by mass, and
the reducing treatment is performed at a reducing temperature of <NUM> or more and <NUM> or less in a state in which the carbonaceous reducing agent is deposited on the surface of the molded mixture so that an amount of the carbonaceous reducing agent as the surface deposit is a proportion of <NUM>% by mass or more and <NUM>% by mass or less when a total amount of the carbonaceous reducing agent required for reducing nickel oxide and iron oxide contained in the mixture without excess or deficiency is taken as <NUM>% by mass.