Valuable metal recovery method

Abstract

To recover a valuable metal from a neutralization leach residue generated at a wet refining process of a nickel oxide ore to reduce a waste amount of the neutralization leach residue.SOLUTION: A valuable metal recovery method is configured to recover a valuable metal including iron from a neutralization leach residue obtained through a wet refining process including: a leaching step of applying a leaching treatment to a nickel oxide ore under high temperature and high pressure by an acid to obtain a leached slurry including a leachate and a leach residue; a neutralization step of neutralizing a free acid included in the leached slurry by a neutralizer; a solid-liquid separation step of solid-liquid separating the neutralized slurry to obtain the leachate and the neutralization leach residue. The valuable metal recovery method includes: a mixing step S21 of mixing the neutralization leach residue and a reducer to obtain a mixture; a reduction step S23 of applying a reduction treatment to the mixture using a rotary hearth furnace at a prescribed temperature to obtain a reduced product; and a recovery step S24 of separating a slag from the reduced product to recover a metal including a valuable metal.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for recovering valuable metals. [Background technology]

[0002] Known methods for smelting nickel oxide ores called limonite or saprolite include a dry smelting process in which nickel matte is produced by roasting together with sulfur in a smelting furnace, a dry smelting process in which nickel is reduced with a carbonaceous reducing agent in a rotary kiln or moving hearth furnace to produce an alloy containing nickel, and a hydrometallurgical process in which nickel and cobalt are leached with sulfuric acid in an autoclave, and a sulfiding agent is added to the leachate to produce a mixed sulfide.

[0003] A hydrometallurgical process using an autoclave allows selective recovery of nickel and cobalt from low-grade nickel oxide ore. For example, Patent Document 1 describes a technique for hydrometallurgical refining of nickel oxide ore, which produces nickel-cobalt mixed sulfides from a leaching slurry obtained by leaching nickel oxide ore. This technique includes a leaching process in which sulfuric acid is added to nickel oxide ore to perform a leaching treatment, and a preliminary neutralization process in which a neutralizing agent is added to the leaching slurry obtained by the leaching treatment to perform a neutralization treatment and remove free acid contained in the leaching slurry, and is characterized in that a nickel-cobalt mixed hydroxide is used as the neutralizing agent in the preliminary neutralization process.

[0004] According to Patent Document 1, by using a specific neutralizing agent in the preliminary neutralization step, it is possible to suppress the variation in pH and perform a stable neutralization process.

[0005] On the other hand, the hydrometallurgical process requires a large amount of sulfuric acid to leach nickel and cobalt, and consumes a large amount of neutralizing agents (limestone and hydrated lime) to neutralize this sulfuric acid, which account for a large proportion of the operating costs.In addition, a large amount of neutralization leaching residue is generated during neutralization leaching.

[0006] When disposing of this neutralization leaching residue, it is buried in a large disposal site called a tailings dam. However, the construction cost of this tailings dam is very high, and depending on the size of the hydrometallurgical processing plant, the amount can exceed 10 billion yen, significantly affecting profitability.

[0007] Incidentally, this neutralization leaching residue contains a large amount of iron, for example, 20 mass % or more. If iron could be efficiently recovered from the neutralization leaching residue, it would be extremely beneficial in terms of both cost and the environment. However, current technology does not allow for efficient recovery of iron from the neutralization leaching residue, and the current situation is that the neutralization leaching residue is disposed of by being buried in a tailings dam, at great cost. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2022-150719 A Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention aims to recover valuable metals from neutralization leaching residue generated during the hydrometallurgical process of nickel oxide ore, thereby reducing the amount of the neutralization leaching residue disposed of. [Means for solving the problem]

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that valuable metals can be efficiently recovered from the neutralization leaching residue by obtaining a mixture of the neutralization leaching residue and a reducing agent and subjecting the mixture to a reduction treatment at a predetermined temperature using a rotary hearth furnace, thereby completing the present invention.

[0011] (1) The first aspect of the present invention is a method for recovering valuable metals, including iron, from the neutralized leaching residue obtained through a hydrometallurgical process including a leaching step of leaching nickel oxide ore with an acid under high temperature and high pressure to obtain a leaching slurry containing a leachate and a leachate residue, a neutralization step of neutralizing free acid contained in the leachate slurry with a neutralizing agent, and a solid-liquid separation step of separating the neutralized slurry into solid-liquid to obtain a leachate and a neutralized leaching residue. The method includes a mixing step of mixing the neutralized leaching residue with a reducing agent to obtain a mixture, a reduction step of reducing the mixture using a rotary hearth furnace to obtain a reduced product, and a recovery step of separating slag from the reduced product to recover metals containing valuable metals. In the reduction step, the mixture is reduced at a reduction temperature of more than 1450°C and not more than 1550°C to obtain the reduced product.

[0012] (2) A second aspect of the present invention is a method for recovering valuable metals according to the first aspect of the present invention, wherein the neutralization leaching residue contains 20% by mass or more of iron.

[0013] (3) A third aspect of the present invention is a method for recovering valuable metals according to the first or second aspect of the present invention, wherein the neutralization leaching residue contains iron in the form of oxide, and in the mixing step, the reducing agent is mixed in such a manner that the mixing ratio is 100 mass% or more when the amount of reducing agent required to reduce the iron oxide contained in the mixture containing the neutralization leaching residue in an appropriate amount is taken as 100 mass%.

[0014] (4) A fourth aspect of the present invention is a method for recovering valuable metals, in which, in any one of the first to third inventions, in the reduction step, the mixture is subjected to a reduction treatment for a reduction time of 3 minutes or more and 50 minutes or less to obtain a reduced product. Effect of the Invention

[0015] According to the present invention, valuable metals such as iron can be efficiently recovered from the neutralization leaching residue generated during the hydrometallurgical process, thereby reducing the amount of the neutralization leaching residue that is disposed of. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a process diagram showing an example of a flow of a hydrometallurgical process for nickel oxide ore. [Diagram 2] FIG. 1 is a process diagram showing an example of a process flow for recovering valuable metals from neutralization leaching residue. [Diagram 3] FIG. 1 is a diagram (plan view) showing an example of the configuration of a rotary hearth furnace. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Specific embodiments of the present invention will be described in detail below. Note that the present invention is not limited to the following embodiments, and various modifications are possible without departing from the gist of the present invention. In this specification, the expression "X to Y" (X and Y are any numerical values) means "X or more and Y or less."

[0018] 1. Overview of the Invention In the present invention, a neutralized leaching residue obtained by subjecting nickel oxide ore to a leaching treatment is used as a raw material, the neutralized leaching residue is mixed with a reducing agent to obtain a mixture, and the mixture is subjected to a reduction treatment using a rotary hearth furnace.

[0019] By separating the slag from the reduced product obtained through such a method, valuable metals such as iron can be recovered in the form of metal.

[0020] The neutralization leaching residue generated during the hydrometallurgical process contains a certain amount of iron. When such a neutralization leaching residue is recovered and valuable metals such as iron are recovered by a hydrometallurgical process, a large amount of acid and neutralizing agent is consumed.

[0021] In contrast, the method for recovering valuable metals according to the present embodiment, in which the neutralized leaching residue is mixed with a reducing agent to obtain a mixture, and the mixture is subjected to a reduction treatment using a rotary hearth furnace, makes it possible to efficiently recover valuable metals such as iron in the form of metal, without consuming large amounts of acid, neutralizing agent, etc., as in the hydrometallurgical process.

[0022] Furthermore, the reduction step is characterized in that the mixture is subjected to a reduction treatment at a reduction temperature of more than 1450°C and not more than 1550°C to obtain a reduced product. When a mixture containing neutralized leaching residue is subjected to a reduction treatment, the mixture shrinks. This makes it possible to reduce the amount of slag in the resulting reduced product, and while valuable metals such as iron are recovered in the form of metal, the volume of slag to be discarded can be reduced. The mixture obtained from the neutralized leaching residue has voids in the mixture. By performing a reduction treatment on such a mixture at a temperature exceeding 1450°C, it is possible to increase the liquid phase ratio of the mixture, thereby improving the fluidity of the mixture, and the voids in the mixture can be significantly reduced. This makes it possible to significantly reduce the voids in the resulting slag, and the volume of slag to be discarded can be reduced. When a reduction treatment is performed on the mixture at a reduction temperature of 1450°C or less, the mixture does not shrink sufficiently, and the volume of slag to be discarded cannot be reduced. If the mixture is subjected to reduction treatment at a reduction temperature exceeding 1550°C, not only will a lot of energy be consumed, but the high temperatures inside the reduction furnace will shorten the lifespan of the furnace hearth and furnace walls, resulting in high maintenance costs and a low furnace operating rate. By performing reduction treatment on the mixture at a reduction temperature between 1450°C and 1550°C, valuable metals such as iron can be efficiently recovered from the neutralization leaching residue while effectively reducing the amount of neutralization leaching residue that is disposed of.

[0023] ≪2. Hydrometallurgical processing≫ First, a hydrometallurgical process for nickel oxide ore, which is a preliminary step of the valuable metal recovery method according to the present embodiment, will be described. The hydrometallurgical process for nickel oxide ore is a hydrometallurgical method for recovering nickel and cobalt by leaching from nickel oxide ore using, for example, a high-temperature pressure acid leaching method (HPAL method).

[0024] Fig. 1 is a process diagram showing an example of the flow of a hydrometallurgical process for nickel oxide ore. As shown in Fig. 1, the hydrometallurgical process for nickel oxide ore includes a leaching step S11 in which nickel oxide ore is leached with an acid under high temperature and high pressure to obtain a leached slurry containing a leachate and a leachate residue, a neutralization step S12 in which free acid contained in the leachate slurry is neutralized with a neutralizing agent, and a solid-liquid separation step S13 in which the neutralized slurry is subjected to solid-liquid separation to obtain a leachate and a neutralized leachate residue.

[0025] [Leaching process] In the leaching step S11, nickel oxide ore is subjected to a leaching treatment with acid under high temperature and high pressure to obtain a leaching slurry containing a leachate and a leach residue. Specifically, using a high-temperature pressurized reaction vessel such as an autoclave, acid is added to a slurry of nickel oxide ore (ore slurry), and the mixture is stirred under conditions of, for example, a temperature of about 230°C to 270°C and a pressure of about 3 MPa to 5 MPa to generate a leachate slurry containing a leachate and a leach residue.

[0026] The raw nickel oxide ore mainly includes so-called laterite ores such as limonite ore and saprolite ore. The nickel content of the laterite ore is usually 0.8% by mass to 2.5% by mass, and is contained as hydroxide or magnesium silicate mineral. The iron content is 10% by mass to 50% by mass, and is mainly in the form of trivalent hydroxide, with some divalent iron being contained in the magnesia siliceous mineral. In addition to such laterite ore, oxide ores containing valuable metals such as nickel, cobalt, manganese, and copper, for example manganese nodules existing on the deep seabed, can be used in the leaching step S11.

[0027] The reactions occurring during the leaching treatment in the leaching step S11 are shown in the following formulas (1) to (5). The following formulas show an example in which sulfuric acid is used as the acid. In the leaching treatment of nickel oxide ore, as shown in the following formulas, a leaching reaction and a high-temperature hydrolysis reaction occur, resulting in the leaching of nickel, cobalt, and the like as sulfates, and the immobilization of the leached iron sulfate as hematite. However, since the immobilization of iron ions does not proceed completely, the liquid portion of the obtained leaching slurry usually contains divalent and trivalent iron ions in addition to nickel, cobalt, and the like.

[0028] Leaching reaction MO+H 2 SO 4 ⇒MSO 4 +H 2 O (1) (In the formula, M represents Ni, Co, Fe, Zn, Cu, Mg, Cr, Mn, etc.) 2Fe(OH) 3 +3H 2 SO 4 ⇒Fe 2 (SO 4 ) 3 +6H 2 O (2) FeO+H 2 SO 4 ⇒FeSO 4 +H 2 O (3) High-temperature hydrolysis reaction 2FeSO 4 +H 2 SO 4 +1 / 2O 2 ⇒Fe 2 (SO 4 ) 3 +H 2 O (4) Fe 2 (SO 4 ) 3 +3H 2 O⇒Fe 2 O 3 +3H 2 SO 4 (5)

[0029] The amount of acid added in the leaching treatment is not particularly limited. For example, when sulfuric acid is used as the acid, it is preferable to use 300 kg to 400 kg per ton of ore. In addition, from the viewpoint of filterability of the leaching residue containing hematite generated in the subsequent solid-liquid separation step S23, it is preferable to adjust the pH of the obtained leachate to 0.1 to 1.0.

[0030] [Neutralization process] In the neutralization step S12, the free acid contained in the leaching slurry obtained in the leaching step S11 is neutralized with a neutralizing agent. In the leaching treatment S11, an excess amount of acid is added in order to improve the leaching rate, so the leaching slurry contains excess acid that was not involved in the leaching reaction, i.e., free acid, and has a low pH. In the neutralization step S12, a neutralizing agent is added to the leaching slurry to adjust the pH to a predetermined range, for example, a range of about pH 2.8 to 3.2.

[0031] This promotes precipitation of iron in the leaching slurry and also enables efficient washing in the next solid-liquid separation step S23.

[0032] The treatment tank for carrying out the neutralization treatment may be, for example, one consisting of only one stage of treatment tank, or one consisting of multiple stages of treatment tanks arranged in series. [Solid-liquid separation process] In the solid-liquid separation step S13, the neutralized slurry obtained in the neutralization step S12 is subjected to solid-liquid separation using a solid-liquid separator such as a thickener to obtain a leachate and a neutralized leach residue (neutralized precipitate).

[0033] Specifically, first, the leaching slurry is diluted with the washing liquid, and then the leaching residue in the leaching slurry is concentrated as a precipitate in the thickener. This reduces the nickel content adhering to the leaching residue according to the degree of dilution. In actual operation, the recovery rate of nickel and cobalt can be improved by connecting thickeners with such functions.

[0034] In addition, solid-liquid separation devices such as thickeners may be connected in multiple stages to wash the neutralized slurry obtained in the neutralization step S12 in multiple stages.

[0035] The overflow liquid obtained from a solid-liquid separator such as a thickener is collected as a leachate. This leachate contains nickel and cobalt as well as impurity elements, and by removing the impurities, it can be made into a mother liquor for nickel recovery.

[0036] On the other hand, the neutralization leaching residue removed in the solid-liquid separation step S13 contains iron in the form of oxide, which is not the target of recovery in the hydrometallurgical treatment. Valuable metals such as iron can be recovered in the form of metal from such neutralization leaching residue by a valuable metal recovery method described below.

[0037] ≪3. How to recover valuable metals≫ Next, a method for recovering valuable metals according to the present embodiment will be described. This method for recovering valuable metals is characterized in that the neutralization leaching residue generated during the above-mentioned hydrometallurgical smelting process of nickel oxide ore is used as a raw material, the neutralization leaching residue is mixed with a reducing agent to obtain a mixture, and the mixture is subjected to a reduction treatment at a predetermined temperature using a rotary hearth furnace.

[0038] By separating the slag from the reduction product obtained by such a method, valuable metals such as iron can be recovered in the form of metal. Furthermore, it is possible to reduce the amount of slag in the reduction product obtained, thereby reducing the volume of slag to be discarded.

[0039] Fig. 2 is a process diagram showing an example of the flow of a process for recovering valuable metals from the neutralization leaching residue of nickel oxide ore. As shown in Fig. 2, this valuable metal recovery method includes a mixing step S21 in which the neutralization leaching residue generated during the hydrometallurgical process is used as a raw material and mixed with a reducing agent to obtain a mixture, a mixture forming step S22 in which the mixture is molded to obtain pellets, a reduction step S23 in which the pellets (mixture) are subjected to a reduction treatment using a rotary hearth furnace to obtain a reduced product, and a recovery step S24 in which slag is separated from the reduced product and metal (valuable metal) is recovered.

[0040] [Mixing process] The mixing step S21 is a step of mixing the raw material powder containing the neutralization leaching residue to obtain a mixture. Specifically, a reducing agent is added to the neutralization leaching residue generated during the above-mentioned hydrometallurgical process of nickel oxide ore, and mixed to obtain a mixture.

[0041] The neutralization leaching residue generated during the hydrometallurgical process is a neutralized precipitate (neutralization leaching residue) separated from the leachate through the leaching step S11, neutralization step S12, and solid-liquid separation step S13 as described above. This neutralization leaching residue contains iron in the form of iron oxide, and for example, the neutralization leaching residue contains 20 mass% or more of iron, and more preferably contains 30 mass% or more of iron.

[0042] The particle size of the neutralization leaching residue contained in the mixture is not particularly limited as long as it does not affect the molding of the mixture. For example, the particle size is preferably 0.01 mm to 10 mm, more preferably 0.01 mm to 5 mm, and even more preferably 0.02 mm to 1 mm.

[0043] The reducing agent is not particularly limited, but may be a carbonaceous reducing agent, such as coal powder, coke, or a reducing agent derived from a plant.

[0044] Among these, the carbonaceous reducing agent preferably contains a reducing agent derived from a plant. If a reducing agent derived from a plant is used, the CO 2Since the reducing agent does not emit any harmful substances, it is possible to reduce the environmental impact. Examples of the reducing agent derived from plants include starch, charcoal, and bamboo charcoal.

[0045] In addition, it is preferable to use a reducing agent having a particle size and particle size distribution similar to that of the raw material neutralization leaching residue, since such a size facilitates uniform mixing and makes it easier for the reduction reaction to proceed uniformly.

[0046] The amount of reducing agent mixed is not particularly limited, but is preferably 100% by mass or more, and more preferably 150% by mass or more and less than 400% by mass, assuming that the amount of reducing agent required to reduce the iron oxides contained in the mixture without excess or deficiency is 100% by mass. When the amount of reducing agent mixed is 100% by mass or more, it becomes possible to reduce the iron oxides more effectively. When the amount of reducing agent mixed is less than 400% by mass, it becomes possible to reduce the iron oxides without the reducing agent being excessive.

[0047] The amount of reducing agent required to reduce the iron oxides contained in the mixture without excess or deficiency is defined as the chemical equivalent required to reduce all of the iron oxides contained in the mixture (pellets) to iron metal in the reduction step S23.

[0048] Table 1 shows the compositions of the neutralization leaching residue and the reducing agent generated during the hydrometallurgical process of nickel oxide ore. However, the compositions of the neutralization leaching residue, etc. are not limited to these.

[0049] [Table 1]

[0050] The iron content of the neutralization leaching residue depends on the type of nickel ore used in the hydrometallurgical treatment, but the iron content of the neutralization leaching residue is often 30% to 65% by mass. The nickel content of the neutralization leaching residue is 0.2% by mass or less, and the magnesium content of the neutralization leaching residue is 0.5% to 3% by mass. Other components often include aluminum and silica at a few mass percent. The particle size of the neutralization leaching residue is often 0.10 mm or less, since it is a residue after hydrometallurgy.

[0051] The mixture may also contain optional additives, such as powders of iron ore, flux components, binders, and the like.

[0052] In the mixing step S21, the raw material powder containing the neutralization leaching residue can be mixed using a mixer or the like. When the raw material powders are mixed to obtain a mixture, the raw material powders may be subjected to a kneading treatment to improve mixability. This applies a shear force to the mixture, which breaks down the agglomerations of the reducing agent, raw material powder, etc., allowing for more uniform mixing, improves the adhesion of each particle, and reduces voids, making it easier to perform a uniform reduction treatment, shortens the reaction time of the reduction reaction, and makes it possible to reduce iron oxides more effectively.

[0053] The kneading treatment can be carried out using a batch kneader such as a Brabender, a Banbury mixer, a Henschel mixer, a helical rotor, a roll, a single-shaft kneader, a twin-shaft kneader, etc. By kneading the mixture, a shear force is applied to the mixture, which breaks down the agglomerations of the reducing agent and raw material powder, etc., allowing for uniform mixing, improving the adhesion of each particle, and reducing voids. This makes it easier for the reduction reaction to occur in the mixture and allows the reaction to occur uniformly, thereby shortening the reaction time of the reduction reaction.

[0054] In addition, after mixing, or after mixing and kneading, the mixture may be extruded using an extruder. This applies pressure (shear force) to the mixture, which breaks down agglomerations of the reducing agent, raw material powder, etc., making the mixture more uniformly mixed. Furthermore, it is possible to reduce voids in the mixture. As a result, it becomes possible to more efficiently reduce the iron oxide in the reduction step S23 described below.

[0055] The extruder is preferably one that can knead and mold the mixture under high pressure and high shear force, and examples of such extruders include single-screw extruders and twin-screw extruders. In particular, an extruder equipped with a twin-screw extruder is preferable. By kneading the mixture under high pressure and high shear force, the agglomeration of the raw material powder mixture can be broken down. In addition, the mixture can be effectively kneaded, and the strength of the mixture can be increased. Furthermore, by using an extruder equipped with a twin-screw extruder, the mixture can be obtained continuously while maintaining high productivity.

[0056] The mixture forming step S22 is a step of forming the mixture of raw material powders obtained in the mixing step S21 into pellets. The mixture forming step is not an essential step in the valuable metal recovery method according to the present embodiment, but forming the mixture into a predetermined shape can improve the handleability of the mixture to be reduced.

[0057] The shape of the pellets is not particularly limited as long as they can be placed on the hearth of the rotary hearth furnace. For example, they can be elliptical, cubic, rectangular, cylindrical, or spherical. Such shapes are simple and not complicated, so molding costs can be reduced. Moreover, the occurrence of defective products can be reduced, and the quality of the obtained molded products can be made uniform, so that a decrease in yield can be suppressed. Furthermore, strength can be easily maintained.

[0058] In particular, the pellet shape is preferably spherical. By making the pellets spherical, the reduction treatment is performed uniformly, and smelting can be performed with less variation and high productivity. When the pellets are spherical, they can be molded to have a diameter of about 10 mm or more and 30 mm or less. When the pellets are rectangular, cubic, cylindrical, or the like, they can be molded to have internal dimensions of about 500 mm or less in length and width.

[0059] The size of the pellet is not particularly limited. For example, the pellet has a volume of 8000 mm 3 It is preferable that the pellet volume is 8000 mm or more. 3 As a result, molding costs are reduced, and the proportion of the surface area of ​​the pellet as a whole is reduced, so that the heating and reducing treatment is uniformly performed, resulting in smelting with less variation and high productivity.

[0060] In the mixture molding step S22, the means is not particularly limited as long as the mixture can be molded into pellets. For example, a molding device such as a briquette device, a pelletizer, or an extruder can be used to mold the mixture into a molded product. As a molding device, for example, a device capable of kneading and molding the mixture under high pressure and high shear force is preferable. By kneading the mixture under high pressure and high shear, it is possible to break up the agglomerations of the mixture, to effectively knead the mixture, and to increase the strength of the resulting molded product.

[0061] It is also possible to use a briquette press for molding. The appropriate device can be selected in consideration of the equipment, strength of the molded product, yield, etc.

[0062] The mixture molding step S22 may include a drying step for drying the molded pellets. The moisture content of the pellets may be excessive, for example, about 50% by mass. When the pellets contain excessive moisture like this, if they are heated suddenly, the moisture inside may evaporate all at once, causing them to expand and break. Therefore, the molded pellets are supplied to a drying step for drying treatment. In the drying step, the pellets are dried so that the solid content is about 60% by mass and the moisture content is about 40% by mass or less. More preferably, the pellets are dried so that the moisture content is about 30% by mass or less.

[0063] By reducing the moisture content of the pellets, it is possible to prevent the pellets from collapsing during the reduction heat treatment in the next reduction step S23, making them easier to remove from the reduction furnace. In addition, since the pellets are often sticky due to excess moisture, drying the pellets makes them easier to handle. Furthermore, in the next reduction step S23, it is possible to prevent moisture from being mixed in the rotary hearth furnace due to the pellets. This makes it possible to more effectively reduce the amount of moisture contained in the atmospheric gas in the rotary hearth furnace, and more effectively prevent oxidation of the metal contained in the reduced product.

[0064] The drying treatment in the drying step is not particularly limited as long as the moisture content of the pellets is about 40% by mass or less, For example, hot air at 70° C. to 400° C. can be blown onto the pellets to dry them.

[0065] In particular, by maintaining the temperature of the pellets at less than 100°C during the drying process, damage to the pellets during the drying process can be suppressed. Furthermore, when drying large-volume pellets, it is acceptable for the pellets to have cracks or breaks before or after drying. When the pellets have a large volume, they melt and shrink during reduction, which often results in cracks or breaks, but the effect of the increase in surface area caused by the cracks or breaks is slight, so this is unlikely to cause a major problem. On the other hand, it goes without saying that if the pellets are designed not to be damaged, the drying process may be omitted.

[0066] The drying process may be carried out continuously at one time or in multiple steps. By carrying out the drying process in multiple steps, the bursting of the pellets can be more effectively suppressed. When the drying process is carried out in multiple steps, the drying temperature from the second step onwards is preferably 150°C or higher and 400°C or lower. By drying within this range, it is possible to dry without the reduction reaction proceeding.

[0067] Table 2 shows an example of the composition (mass %) of the solid content excluding the carbon content in the pellets after drying treatment, although the composition of the pellets is not limited to this.

[0068] [Table 2]

[0069] [Reduction process] In the reduction step S23, the pellets (mixture) obtained in the mixture molding step S22 are subjected to a reduction treatment. Specifically, the obtained pellets (mixture) are placed on the hearth of a reduction furnace, and the pellets (mixture) are heated to a predetermined temperature in the reduction furnace, thereby performing a reduction heat treatment. This reduction heat treatment produces a reduced product containing iron metal, which is a valuable metal, and slag. In the following, the pellets to be subjected to the reduction treatment are referred to as a mixture for convenience.

[0070] Here, in the reduction step S23, a rotary hearth furnace with a rotating hearth is used as the reduction furnace to perform the reduction treatment. The rotary hearth furnace has a high temperature range inside the furnace because the atmospheric gas circulates inside the furnace, and has better thermal efficiency than other reduction furnaces such as rotary kilns. In addition, other reduction furnaces such as rotary kilns may be mixed with air because gas is forcibly drawn out from the furnace and discharged, making it difficult to control the oxygen concentration inside the furnace. In addition, in a rotary furnace such as a rotary kiln, the sample moves downward as the furnace rotates, so it is not possible to uniformly flow gas inside the furnace, and it may be difficult to apply heat to the sample uniformly.

[0071] In contrast, a rotary hearth furnace is highly airtight, and the rotating hearth makes it possible to apply heat uniformly to the sample as it moves through a tunnel from the charging port to the discharge port. Therefore, by performing reduction treatment on a mixture using a rotary hearth furnace with a rotating hearth, it is possible to efficiently reduce iron oxides, and to efficiently recover valuable metals such as iron metal.

[0072] Fig. 3 is a diagram (plan view) showing an example of the configuration of a rotary hearth furnace with a rotating hearth. As shown in Fig. 3, a rotary hearth furnace 2 can be used that is circular and divided into a plurality of treatment chambers 20a to 20d. In the rotary hearth furnace 2, each treatment is carried out in each region while rotating in a predetermined direction. In this rotary hearth furnace, the treatment temperature in each region can be adjusted by controlling the time (travel time, rotation time) required to pass through each region, and the mixture is heated and reduced every time the rotary hearth furnace rotates once.

[0073] The heating means of the reduction furnace (rotary hearth furnace) is not particularly limited, and may be a burner or may be electricity. When a reduction furnace (rotary hearth furnace) having a burner is used, for example, LPG, LNG, coal, coke, pulverized coal, etc. are used as fuel. The cost of these fuels is very low, and the equipment costs and maintenance costs can be kept significantly lower than those of electric furnaces, etc.

[0074] When using a reduction furnace (rotary hearth furnace) having a burner, it is preferable to set the air-fuel ratio of the burner to less than 1.0. When performing reduction treatment by heating using a burner, air is supplied to the burner together with fuel. Here, since unreacted oxygen is contained in the combustion gas, oxygen may be mixed into the atmospheric gas in the reduction furnace (rotary hearth furnace) and the oxygen concentration may increase. Then, the obtained iron metal may be oxidized again, and the iron oxide may not be efficiently reduced. By setting the air-fuel ratio of the burner to less than 1.0, it is possible to reduce the amount of oxygen supplied into the reduction furnace, which reduces the oxygen concentration in the reduction furnace and makes it possible to efficiently reduce the iron oxide.

[0075] It is also preferable to carry out an operation to reduce the oxygen concentration in the reduction furnace. For example, the oxygen concentration in the reduction furnace may be reduced by replacing the atmosphere in the reduction furnace with an inert gas. Examples of the inert gas include nitrogen and argon gas.

[0076] The oxygen concentration in the reducing furnace is not particularly limited, but is preferably 0.1 mass% or less. The oxygen concentration in the reducing furnace can be controlled by controlling the air-fuel ratio of the burner as described above or by performing an operation to reduce the oxygen concentration in the reducing furnace.

[0077] In the reduction process, the mixture is reduced at a reduction temperature of more than 1450°C and less than 1550°C. If the mixture is reduced at a reduction temperature of less than 1450°C, the mixture will not shrink sufficiently, and the volume of slag to be discarded cannot be reduced. If the mixture is reduced at a reduction temperature of more than 1550°C, not only will a lot of energy be consumed, but the high temperature inside the reduction furnace will shorten the life of the hearth and furnace walls of the reduction furnace, resulting in high maintenance costs and a low operating rate of the reduction furnace. By reducing the mixture at a reduction temperature of more than 1450°C and less than 1550°C, valuable metals such as iron can be efficiently recovered from the neutralization leaching residue, while effectively reducing the amount of the neutralization leaching residue to be discarded.

[0078] The reduction temperature in the reduction step is preferably 1460°C or higher and 1540°C or lower, and more preferably 1470°C or higher and 1530°C or lower.

[0079] The reduction time in the reduction step is not particularly limited, but is preferably 3 minutes or more and 50 minutes or less, more preferably 6 minutes or more and 40 minutes or less, and even more preferably 10 minutes or more and 35 minutes or less. By setting the reduction time to 3 minutes or more, it is possible to allow the reduction reaction to proceed sufficiently, and iron metal can be obtained efficiently. By setting the reduction time to 50 minutes or less, iron metal can be obtained in a short time.

[0080] In the reduction step S23, the reducing agent may be additionally added during the reduction treatment. Note that "during the reduction treatment" refers to the period from the start of the reduction reaction of the iron oxide in the mixture until the end of the heat treatment of the mixture.

[0081] The proportion of the reducing agent to be added additionally is not particularly limited. For example, when the reducing agent contained in the mixture is taken as 100 mass%, it is preferable to add the reducing agent in a range of about 1 mass% to 30 mass%. By adding the reducing agent additionally in such a range, it becomes possible to reduce the iron oxide more effectively. Moreover, it is possible to prevent over-reduction.

[0082] [Recovery process] In the recovery step S24, slag is separated from the reduced product produced in the reduction step S23 to recover iron metal, which is a valuable metal. Specifically, the valuable metal phase is separated and recovered from a mixture (reduced product) containing a valuable metal phase (solid phase of valuable metal) and a slag phase (solid phase of slag) obtained by reduction heat treatment of the mixture.

[0083] In the recovery step S24, the recovery method is not particularly limited as long as the valuable metal can be separated and recovered from the reduced material. For example, a method of cooling the mixture (reduced material) and separating and recovering the valuable metal phase by sieving the mixture of valuable metal phase and slag phase obtained as a solid, a method of separating and recovering the valuable metal phase by specific gravity, or a method of separating and recovering the valuable metal phase by magnetic force can be used. In this case, the mixture (reduced material) obtained as a solid may be subjected to a pulverization treatment in advance. Note that the valuable metal phase may be separated and recovered from the reduced material by combining these methods.

[0084] In particular, in the valuable metal recovery method according to the present embodiment, valuable metals having a large average particle size can be obtained, so that the valuable metal phase and the slag phase can be easily separated by cooling the mixture (reduced material) and applying an impact to the reduced material obtained as a solid. Moreover, the valuable metal phase and the slag phase obtained have poor wettability, so that they can be easily separated. Examples of such a vibration application method include a method in which the reduced material is dropped across a predetermined drop, or a method in which a predetermined vibration is applied during sieving.

[0085] As described above, the neutralization leaching residue generated during the hydrometallurgical process of nickel oxide ore is used as a raw material, and a reducing agent is mixed to obtain a mixture, which is then subjected to a reduction treatment at a predetermined temperature using a rotary hearth furnace, thereby making it possible to efficiently recover iron metal. This makes it possible to effectively utilize the neutralization leaching residue instead of disposing of it, and significantly reduces the amount of neutralization leaching residue landfilled. EXAMPLES

[0086] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples in any way.

[0087] [Mixing process] Neutralized leaching residue generated during the hydrometallurgical process of limonite, a nickel oxide ore, and the reducing agents shown in Table 3 were prepared, and mixed in a mixer while adding an appropriate amount of water to prepare a mixture.

[0088] The iron content of the neutralization leaching residue was 41% by mass, the magnesium content was 1.9% by mass, and the silica content was 2.7% by mass. The average particle size was 35 μm. A laser diffraction particle size distribution analyzer was used to measure the average particle size of the neutralization leaching residue. Coal, coke, bamboo charcoal, wood charcoal, and starch were used as reducing agents, as shown in Table 3.

[0089] (Reducing agent mixing ratio) The reducing agent was mixed in such a manner that the amount of reducing agent required to reduce the iron oxides in the mixture (neutralized leaching residue) to be mixed in the correct amount was 290% by mass, assuming that the amount of reducing agent required was 100% by mass.

[0090] [Mixture forming process] The mixture was subjected to a mixture molding process to form cylindrical pellets. A pelletizer was used for molding. The obtained pellets were subjected to a sieving process to form pellets with a diameter of 15±2 mm.

[0091] The prepared pellets were dried by blowing hot nitrogen air at 150° C. to 200° C. so that the solid content was about 71% by mass and the moisture content was about 29% by mass.

[0092] [Reduction process] Next, the pellets obtained in the mixture molding process were subjected to a reduction treatment. Specifically, the pellets after the drying treatment were charged into a rotary hearth furnace with a rotating hearth. The temperature of the rotary hearth furnace, which is a reduction furnace, at the time of charging was set to 500±20°C. In addition, the inside of the rotary hearth furnace was replaced with nitrogen, so that the atmospheric gas in the rotary hearth furnace was made substantially oxygen-free.

[0093] The hearth of the rotary hearth furnace was covered with alumina particles, and the pellets were placed on top of the alumina particles to suppress the reaction between the pellets and the hearth. The reduction treatment conditions for the pellets were the reduction temperature and reduction time shown in Table 3. After the reduction treatment, the pellets were quickly cooled to room temperature in a nitrogen atmosphere and removed from the rotary hearth furnace.

[0094] The iron metal ratio and iron recovery ratio of the reduced product taken out of the furnace were measured. The measurements were performed using an ICP emission spectrometer (SHIMAZU S-8100 model).

[0095] The iron metallization rate was calculated based on the measured values ​​using the following formula.

[0096] (iron metallization rate) Iron metalization rate (%) = (amount of iron metal in the mixture / total amount of iron in the mixture) x 100

[0097] [Recovery process] Next, iron metal, which is a valuable metal, was recovered from the reduced material obtained by the reduction treatment. Specifically, the reduced material was subjected to a pulverization treatment, and then slag containing non-magnetic substances such as magnesium oxide and silica was separated by magnetic separation, and the magnetic iron metal was recovered.

[0098] The iron recovery rate was calculated from the raw material content in the pellet stack charged into the reduction furnace, the iron content in the raw material, and the amount of recovered iron metal. The iron recovery rate was calculated using the following formula.

[0099] (Iron recovery rate) Iron recovery rate (%) = {mass of recovered iron metal / (mass of charged pellets x iron content in pellets)} x 100 (%)

[0100] [Table 3]

[0101] As shown in Table 3, in Examples 1 to 9, in which a neutralization leaching residue generated during the hydrometallurgical process of nickel oxide ore was mixed with a reducing agent to obtain a mixture, and the mixture was then subjected to a reduction treatment at a reduction temperature of more than 1450°C and not more than 1550°C using a rotary hearth furnace, iron metal with a high iron metallization rate and iron recovery rate was obtained.

[0102] The shrinkage rate of the pellets was measured by measuring the volume of the reduced material obtained by these reduction treatments. In Examples 1 to 9, in which the mixture was subjected to reduction treatment at a reduction temperature of more than 1450°C and not more than 1550°C using a rotary hearth furnace, the shrinkage rate of the pellets was 50% by volume or more. The reason why the shrinkage rate of the pellets was 50% by volume or more in these Examples is believed to be that the reduction temperature was within the range of the present invention, so that an appropriate amount of liquid phase was generated and the voids in the pellets could be greatly reduced.

[0103] On the other hand, in Comparative Example 1, in which the mixture was subjected to reduction treatment at a reduction temperature of 1450°C or less using a rotary hearth furnace, the shrinkage rate of the pellets was less than 50% by volume. This confirmed that by subjecting the mixture to reduction treatment at a reduction temperature of more than 1450°C, valuable metals such as iron can be efficiently recovered from the neutralization leaching residue while the amount of the neutralization leaching residue to disposal can be effectively reduced.

[0104] In Comparative Example 2, in which the mixture was subjected to reduction treatment at a reduction temperature of over 1550°C using a rotary hearth furnace, the pellet shrinkage rate was 50% by volume or more, but the iron metal with a higher iron metalization rate and iron recovery rate compared to Examples 1 to 9 was not obtained. Therefore, the recovery rate of valuable metals was not increased by increasing the reduction temperature. Furthermore, in Comparative Example 2, the temperature inside the reduction furnace was high, which increased the maintenance cost and reduced the operating rate of the reduction furnace. Therefore, it is presumed that if the mixture is subjected to reduction treatment at a reduction temperature of over 1550°C using a rotary hearth furnace, valuable metals cannot be recovered efficiently. [Explanation of symbols]

[0105] 1 charging port 2 Rotary hearth furnace 20a~20d Processing chamber 21 Preheating Room 22 Cooling room (discharge port)

Claims

1. A method for recovering valuable metals, comprising: a leaching step of leaching nickel oxide ore with an acid under high temperature and high pressure to obtain a leaching slurry containing a leachate and a leachate residue; a neutralization step of neutralizing free acid contained in the leachate slurry with a neutralizing agent; and a solid-liquid separation step of separating the neutralized slurry into solid and liquid to obtain a leachate and a neutralized leachate residue, the method comprising the steps of: recovering valuable metals including iron from the neutralized leachate residue obtained through a hydrometallurgical process, the method comprising the steps of: a mixing step of mixing the neutralized leaching residue with a reducing agent to obtain a mixture; a reduction step of subjecting the mixture to a reduction treatment using a rotary hearth furnace to obtain a reduced product; a recovery step of separating slag from the reduced material and recovering metals including valuable metals; having In the reduction step, the mixture is subjected to a reduction treatment at a reduction temperature of more than 1450° C. and not more than 1550° C. to obtain a reduced product. Methods for recovering valuable metals.

2. The neutralized leaching residue contains 20% by mass or more of iron. The method for recovering valuable metals according to claim 1.

3. The neutralized leach residue contains iron in the form of an oxide; In the mixing step, the reducing agent is mixed in such a mixing ratio that the reducing agent is 100% by mass or more, assuming that the amount of the reducing agent required for reducing the iron oxide contained in the mixture containing the neutralization leaching residue in an appropriate amount is 100% by mass. The method for recovering valuable metals according to claim 1 or 2.

4. In the reduction step, the mixture is subjected to a reduction treatment for a reduction time of 3 minutes or more and 50 minutes or less to obtain a reduced product. The method for recovering valuable metals according to claim 1 or 2.

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