JP7607238B2 - Metal oxide reduction device and its use - Google Patents

Description

The present invention relates to a reduction device that can continuously produce reduced metal oxides by reducing metal oxides with hydrogen, and its uses.

The construction of a lunar base is one of the most important projects in space development, and water and oxygen are essential for manned activities on the lunar surface. For this reason, attention is being paid to the development of a process for producing water and oxygen by hydrogen reduction of metal oxides such as ilmenite (iron ore), which is a lunar resource. In addition, hydrogen reduction technology using a similar process can also be applied to the production of oxygen-deficient metal oxides by reducing metal oxides on Earth. Oxygen-deficient metal oxides have attracted attention as highly functional materials and are expected to be applied in a wide range of fields. A known method for hydrogen reduction of metal oxides is to bring hydrogen molecules into contact with the metal oxide at temperatures of several hundred degrees Celsius, but this method is prone to temperature unevenness in the metal oxide, which causes the reduction process to proceed unevenly, making it difficult to efficiently produce the desired oxygen-deficient metal oxides with a high conversion rate.

Therefore, there is a need for a method for efficiently producing oxygen-deficient metal oxides by hydrogen reduction, and JP 2020-37491 A (Patent Document 1) discloses a method for producing oxygen-deficient metal oxides, which comprises filling a fluidized bed reactor with a metal oxide that exhibits a Gibbs free energy change of 100 kJ/mol or less in the reduction temperature range, heating the fluidized bed reactor to a predetermined reduction temperature, continuously supplying a reducing gas containing a predetermined concentration of hydrogen gas to the fluidized bed reactor for a predetermined period of time, and discharging the generated water vapor from the fluidized bed reactor.

JP 2020-37491 A

However, the production method using a fluidized bed reactor in Patent Document 1 does not allow for continuous production of oxygen-deficient metal oxides, and is difficult to scale up, resulting in low industrial productivity. The uniformity of reduction is also insufficient, and for example, it is difficult to produce a reduced product by reducing a metal oxide with a large Gibbs free energy change at a high conversion rate.

The object of the present invention is therefore to provide a reduction device capable of producing reduced metal oxides with high productivity, and its use.

As a result of intensive research into achieving the above object, the inventors discovered that it is possible to produce reduced metal oxides with high productivity by moving the metal oxide particles while stirring them and reducing them with the reducing gas while circulating a reducing gas in a direction opposite to the direction of movement of the metal oxide particles, and thus completed the present invention.

That is, the reduction apparatus of the present invention is a reduction apparatus including a reaction unit having a reactor for reducing metal oxide particles with hydrogen gas and a circulation means for circulating a reducing gas containing the hydrogen gas through the reactor,
the reactor has a supply port for supplying the metal oxide particles;
an outlet for recovering a reduced product obtained by reducing the metal oxide particles;
a stirring and moving means for stirring and moving the metal oxide particles supplied from the supply port to the outlet in the presence of the reducing gas;
an inlet for introducing the reducing gas;
and an outlet for recovering or discharging exhaust gas after the reaction;
The flow means is a flow means for flowing the reducing gas in a direction opposite to a moving direction of the metal oxide particles.

The reactor may have a cylindrical space as a reaction chamber, and the stirring and moving means may be a screw housed in the cylindrical space. The outer diameter of the screw may be 90% or more of the inner diameter of the reactor. The reaction unit may further include a heating means for heating the reactor. The inlet may be formed downstream of the region heated by the heating means in the moving direction of the metal oxide particles. The reducing gas may be hydrogen gas alone, or a mixed gas of hydrogen gas and an inert gas. The metal oxide particles may be reduced at a temperature of 853 to 1323 K and a pressure of 150 to 500 kPa. The average particle size of the metal oxide particles may be 1 to 1000 μm. The reduced body may be an oxygen-deficient metal oxide particle. The reduction device may further include a circulation line for circulating the reducing gas contained in the exhaust gas to the reactor. The reduction device may further include an electrolysis unit for electrolyzing water contained in the exhaust gas into hydrogen gas and oxygen gas, and circulate the hydrogen gas generated in the electrolysis unit to the reactor.

The present invention also includes a method for continuously producing a reduced metal oxide by using the reduction device to move the metal oxide particles supplied from the supply port to the outlet while stirring, while circulating the reducing gas in a direction opposite to the direction of movement of the metal oxide particles, and recovering the reduced metal from the outlet. In this method, the reactor may be cylindrical, and the stirring and moving means may be a screw built into the cylinder, and the metal oxide particles may be moved while being stirred by the rotation of the screw.

The present invention also includes a method for controlling the degree of oxygen deficiency in the metal oxide by adjusting at least one condition selected from the group consisting of the hydrogen gas concentration in the reducing gas, the flow rate of the reducing gas, the temperature, the pressure, the stirring speed, and the reduction time, in which the metal oxide particles supplied from the supply port are moved to the outlet while being stirred, and the reduced product is recovered from the outlet, using the reduction device.

In the present invention, the metal oxide particles are stirred and moved while the reducing gas is circulated in the direction opposite to the movement of the metal oxide particles, and are reduced by the reducing gas. This allows the metal oxide reduction product to be produced continuously with a high conversion rate, and industrial productivity is high. In addition, the degree of oxygen deficiency in the metal oxide can be easily controlled by adjusting various conditions. Furthermore, the use of an electrolysis unit can produce not only water as a by-product, but also oxygen, making it effective for reducing metal oxides on the lunar surface.

FIG. 1 is a schematic diagram for explaining an example of a reduction device of the present invention. FIG. 2 is an enlarged view for explaining a screw housed inside the reactor in the reduction apparatus of FIG. FIG. 3 is a graph showing the state of reduction of tungsten oxide using the reduction device of Example 1. FIG. 4 is a scanning electron microscope photograph of tungsten oxide particles, which are the raw material used in Example 1. FIG. 5 is a scanning electron microscope photograph of the oxygen-deficient tungsten oxide particles obtained in Example 1.

Below, an embodiment of the present invention will be described in detail, with reference to the attached drawings as necessary.

In the example shown in FIG. 1, the metal oxide reduction device 1 is equipped with a cylindrical reactor 4 for reducing metal oxide particles 2 with hydrogen gas. One end of the cylindrical reactor 4 is formed with a supply port 4a for supplying the metal oxide particles 2 to a reaction chamber, which is the internal space of the cylindrical reactor 4, and the metal oxide particles 2 are supplied from this supply port 4a to the reaction chamber by a hopper 3, which is a feeder of the metal oxide particles 2.

As shown in FIG. 2, the cylindrical reactor 4 has a screw 5 built into the cylindrical interior as an agitation and movement means. The screw 5 is in the shape of a propeller with blades 5b formed in a spiral shape around the shaft 5a, and has a structure in which the blades 5b can be rotated around the shaft 5a by the drive unit 6. When the metal oxide particles 2 are supplied to the reaction chamber from the supply port 4a while being rotated by the drive unit 6, the screw 5 moves the metal oxide particles 2 to the discharge port 4b formed at the other end of the cylindrical reactor 4 while stirring the metal oxide particles 2 with the agitation and propulsion force of the blades 5b.

Between the supply port 4a and the discharge port 4b, a heater 7 is disposed as a heating means for heating the cylindrical reactor 4. The heater 7 heats the reaction chamber of the cylindrical reactor 4, thereby allowing the reduction of the metal oxide particles 2 to proceed. That is, in the reaction chamber of the cylindrical reactor 4, the area heated by the heating means becomes the main reaction chamber for the reduction reaction of the metal oxide particles 2, and the reduced product 2a reduced in the reaction chamber is collected in a collection container 8 from the discharge port 4b.

The reducing gas for reducing the metal oxide particles 2 is introduced from an inlet 4c formed downstream of the outlet 4b in the moving direction of the metal oxide particles 2, and is circulated in the reaction chamber of the reactor 4. The reducing gas introduced from the inlet 4c flows in a countercurrent manner against the metal oxide particles 2 moving toward the outlet 4b by the screw 5. That is, the reducing gas flows in the reaction chamber of the cylindrical reactor 4 in a direction opposite to the moving direction of the metal oxide particles 2, and comes into contact with the metal oxide particles 2 moving while being stirred, thereby reducing the metal oxide particles 2. In the present invention, the reducing gas flows in a countercurrent manner against the metal oxide particles 2 moving while being stirred by the screw 5, so that the reducing gas can be uniformly brought into contact with the metal oxide particles 2, and the metal oxide particles can be uniformly reduced with a high conversion rate.

The reducing gas is a mixture of hydrogen gas and argon gas, and is introduced into the inlet 4c through the reducing gas supply line 11. The reducing gas supply line 11 is a supply line formed by joining a hydrogen gas supply line 9a connected to a hydrogen cylinder 9 and an argon gas supply line 10a connected to an argon cylinder 10. The hydrogen gas supply line 9a and the argon gas supply line 10a are each equipped with a flow control device (mass flow controller) 9b, 10b, and the ratio of hydrogen gas to argon gas can be controlled by adjusting the flow rate of each gas.

After the reduction gas reduces the metal oxide particles 2 in the reaction chamber, it becomes exhaust gas. A part of the exhaust gas after the reaction is discharged through the exhaust line 12 from the exhaust port 4d formed between the region of the reaction chamber heated by the heating means (the region where the heating means is arranged) and the supply port 4a, but the remaining exhaust gas enters the supply machine 3. Therefore, an exhaust port 3a for discharging the exhaust gas is also formed at the top of the supply machine 3, and the exhaust gas remaining inside the supply machine 3 is discharged through the exhaust line 13 from the exhaust port 3a. The exhaust gas that passes through the exhaust line 12 and the exhaust line 13 join together in the exhaust line 14 and is collected in the condenser 16. Furthermore, the exhaust line 14 is equipped with a moisture meter 15, which can measure the amount of moisture in the exhaust gas. In the present invention, water (water vapor) generated by reduction is continuously discharged outside the reaction system, and a non-equilibrium state is maintained within the reaction chamber, making reduction possible even in systems where a reaction is thermodynamically difficult, and even difficult-to-reduc metal oxide particles can be easily reduced to the desired level of oxygen deficiency.

In the condenser 16, the recovered exhaust gas is cooled and separated into reducing gas and water. The separated water passes through a water supply line 17 and is recovered in an electrolysis unit 19, and the separated reducing gas passes through a reducing gas supply line 18 as a circulation line, merges with the reducing gas supply line 11, and is introduced into the cylindrical reactor 4 via an inlet 4c for circulation.

In the electrolysis unit 19, the recovered water is electrolyzed into hydrogen gas and oxygen gas. The hydrogen gas produced by electrolysis is circulated by joining the reducing gas supply line 11 through the hydrogen gas supply line 20, which serves as a circulation line, and introducing it into the cylindrical reactor 4 through the inlet 4c.

The reduction device of the present invention is not limited to the reduction device shown in FIG. 1. Details of each unit and raw material used in the reduction device and reduction method of the present invention are described below.

[Reaction unit]
The reaction unit is not particularly limited as long as it includes a reactor for reducing metal oxide particles with hydrogen gas and a distribution means for circulating a reducing gas containing the hydrogen gas through the reactor, and the metal oxide particles can be moved while being stirred and reduced by the reducing gas while the reducing gas is circulated in a direction opposite to the direction of movement of the metal oxide particles.

(Reactor)
The internal shape of the reactor (the internal space as the reaction chamber) may be selected according to the shape of the stirring and moving means as long as it can move the metal oxide particles while stirring them. In addition to a cylindrical shape, the reactor may be in a disk shape, rectangular shape, or the like. However, a cylindrical shape is preferred from the viewpoint of efficiently and uniformly reducing the metal oxide particles.

The external shape of the reactor is not limited to a shape corresponding to the internal shape, and may be different from the internal shape, but from the standpoint of productivity, etc., it is preferable for the external shape to be the same as the internal shape. Therefore, the reactor shape is preferably cylindrical.

The locations of the supply inlet, outlet, inlet, and outlet are not particularly limited, so long as they are positions that allow the reduction gas to flow in a countercurrent manner relative to the moving metal oxide particles.

When the reactor is cylindrical, in FIG. 1, the inlet for the reducing gas is formed downstream of the outlet for the metal oxide particles in the direction of movement of the metal oxide particles, but this is not particularly limited and it may be formed upstream. However, from the viewpoint of improving the efficiency and uniformity of the reduction, it is preferable to form the inlet downstream of the area where the heating means is disposed, and it is even more preferable to form the inlet near the outlet, and even more preferable to form it downstream of the outlet.

In addition, in FIG. 1, the exhaust gas outlet is formed between the region where the heating means is disposed and the supply port, but this is not particularly limited and may be formed upstream of the supply port in the direction of movement of the metal oxide particles. However, from the viewpoint of improving the efficiency and uniformity of the reduction, it is preferable to form it upstream of the region where the heating means is disposed, and from the viewpoint of efficiently recovering or discharging the exhaust gas, it is preferable to form it between the region where the heating means is disposed and the supply port.

(Agitation and movement means)
The stirring and moving means is not particularly limited as long as it can move the metal oxide particles from the supply port to the discharge port while stirring them, and is not limited to the screw shown in Fig. 1, but may be, for example, a vibrating conveyor, an eccentric rotary pump, a vibrating staircase (a step-like device capable of continuously moving the metal oxide particles to the lower step by vibration), etc. Among these, the screw is preferred because it can easily reduce the metal oxide particles uniformly.

The shape and size of the screw may be selected according to the characteristics of the target reduced product and is not particularly limited, but may be selected as follows from the viewpoint of uniform reduction of the metal oxide particles:

The shape of the screw is not particularly limited as long as it is a propeller shape with spiral blades centered on the axis, and the screw diameter on the upstream side may be different from the screw diameter on the downstream side, but a shape in which the screw diameter on the upstream side is the same as the screw diameter on the downstream side is preferable in order to uniformly stir the metal oxide particles.

The depth of the screw groove (height of the blade relative to the shaft) (t) may be 5% or more of the outer diameter of the screw (screw diameter D), for example, 10 to 80%, preferably 20 to 60%, more preferably 30 to 50%, and even more preferably 35 to 40%. If the groove depth is too shallow, there is a risk of reduced productivity.

The screw pitch P can be selected from the range of about 0.1 to 5 times the screw diameter D, for example, 0.3 to 3 times, preferably 0.5 to 2 times, more preferably 0.8 to 1.5 times, more preferably 1 to 1.4 times, and most preferably 1.1 to 1.3 times. If the ratio of the pitch to the outer diameter is too small, there is a risk that the productivity of the reduced material will decrease, and if it is too large, there is a risk that it will be difficult to uniformly stir the metal oxide particles.

The inclination angle of the blades relative to the axis of the screw may be 10° or more, for example, 20 to 80°, preferably 40 to 80°, and more preferably 60 to 70°. If the inclination angle is too small, the productivity of the reduced material may decrease, and it may become difficult to uniformly stir the metal oxide particles.

The screw diameter D may be 80% or more (particularly 90% or more) of the inner diameter of the reaction chamber (cylindrical space) in the cylindrical reactor, for example 80 to 99%, preferably 90 to 98%, further preferably 92 to 97%, and even more preferably 93 to 95%. If the ratio of the screw diameter D to the inner diameter of the reaction chamber is too small, there is a risk that the metal oxide particles will not be stirred uniformly.

The screw rotation speed can be appropriately selected depending on the desired degree of oxygen deficiency, and may be 0.1 rpm or more (particularly 0.4 rpm or more), for example, 0.1 to 11 rpm, preferably 0.3 to 3.0 rpm, and more preferably 0.4 to 1.1 rpm. If the rotation speed is too low, it may be difficult to uniformly stir the metal oxide particles.

In the present invention, the reduction time and the degree of oxygen deficiency can be adjusted by adjusting the size and shape of the screw and the rotation speed. In particular, since the rotation speed is inversely proportional to the reduction time, the reduction time can be easily adjusted by adjusting the rotation speed. Furthermore, since increasing the rotation speed also increases the stirring effect and makes it easier to reduce uniformly, under conditions where the reaction time is short, the uniformity of the reduction can be easily improved by increasing the rotation speed. Furthermore, when a long reaction time is required, it is also possible to adjust the stirring effect to a long time by selecting the shape of the screw.

The reduction time can be selected from a range of about 1 minute or more by adjusting the stirring and moving means as described above depending on the type of target reductant (e.g., the degree of difficulty of reduction). A specific reduction time may be, for example, 1 to 120 minutes, preferably 3 to 60 minutes, and more preferably 5 to 30 minutes.

(Heating Means)
The heating means is not particularly limited as long as it can heat the reaction chamber to the desired temperature, and may be a conventional heater such as an electric furnace or a combustion furnace.

The heating means is disposed in a predetermined region of the reactor and forms the main reaction chamber. The heating temperature by the heating means can be appropriately selected depending on the type of target reductant, but can be selected from the range of, for example, about 753 to 1523 K, preferably 853 to 1323 K, and more preferably 1023 to 1273 K.

In the present invention, the degree of oxygen deficiency of the resulting reduced product can be adjusted by adjusting the heating temperature with the heating means, and the degree of oxygen deficiency of the reduced product can be easily adjusted by combining this with other conditions such as the reduction time with the stirring and moving means.

(Means of distribution)
The distribution means is not particularly limited as long as it can distribute the reducing gas countercurrently against the moving metal oxide particles, but from the viewpoint of simplicity, a means combining a cylinder filled with compressed reducing gas and a flow control device (mass flow controller) as shown in Fig. 1 is preferred. When the reducing gas is a mixed gas of hydrogen gas and an inert gas, the ratio of the mixed gas can be easily adjusted by controlling the flow rate of the gas supplied from each cylinder with a flow control device.

The pressure in the reactor may be adjusted by adjusting the flow means. The pressure in the reactor can be appropriately selected depending on the type of the target reductant, and can be selected, for example, from the range of about 100 to 1000 kPa, preferably 150 to 500 kPa, and more preferably 200 to 400 kPa.

The flow means may further include an automatic control means for automatically controlling the flow control device. The automatic control means may automatically control the flow control device to control the flow rate during the reaction, or may automatically control the flow rate according to the target reduced product. The automatic control means may be linked to the heater and/or the stirring and moving means, and by linking them, the heating temperature, pressure, stirring conditions, and reduction time in the reactor can be automatically controlled simultaneously.

[Supply unit and recovery unit]
The reduction apparatus of the present invention may further include, in addition to the reaction unit, a supply unit for supplying metal oxide particles as a raw material and a recovery unit for recovering the reduced product. Both the supply unit and the recovery unit can use a conventional supply device and recovery container, and a hopper is generally used as the supply device because it can easily supply continuously.

[Condensation unit]
In the reduction apparatus of the present invention, the reaction unit is provided with a stirring and moving means and a circulating means, so that the metal oxide particles can be continuously reduced. Therefore, the reduction apparatus of the present invention is not limited to the reduction apparatus of Fig. 1 equipped with a condenser as a condensation unit, and may be a reduction apparatus that does not have a condensation unit. In other words, the exhaust gas generated by the reduction reaction in the reactor may be discharged or may be recovered in the condensation unit and reused for reduction.

In a reduction device equipped with a condensation unit, unreacted hydrogen gas can be reused, improving energy efficiency. In other words, the exhaust gas generated in the reactor contains a reducing gas containing unreacted hydrogen gas and water vapor. Therefore, the condensation unit can separate the exhaust gas into reducing gas and water, and by returning the separated reducing gas to the reactor via a circulation line, the unreacted hydrogen gas can be reused, improving energy efficiency. A conventional condenser can be used as the condensation unit.

[Electrolysis unit]
As described above, the reduction device of the present invention can continuously produce a reduced substance by the reaction unit, and therefore the condensation unit is not an essential component. Therefore, the reduction device of the present invention is not limited to the reduction device of Fig. 1 equipped with a condensation unit and an electrolysis unit, and may be a reduction device not equipped with an electrolysis unit, for example, a reduction device not equipped with a condensation unit and an electrolysis unit, or a reduction device equipped with a condensation unit but not equipped with an electrolysis unit.

On the other hand, in a reduction device equipped with a condensation unit and an electrolytic decomposition unit, the water separated in the condensation unit can be decomposed into hydrogen gas and oxygen gas, so the hydrogen gas separated in the electrolytic decomposition unit can be returned to the reactor via a circulation line, allowing the water vapor in the exhaust gas to be reused. A conventional electrolysis device can be used as the electrolysis unit. Furthermore, the electrolysis unit also generates oxygen gas, making it effective for reducing lunar soil on the lunar surface, where the supply of oxygen gas is important. Furthermore, since the supply of water is important as well as oxygen gas on the lunar surface, it is also possible to simultaneously generate water and oxygen gas by supplying some of the water generated in the condensation unit to the electrolysis unit.

[Raw materials]
(Metal oxide particles)
In the reduction device of the present invention, even particles formed of difficult-to-reduc metal oxides can be uniformly reduced, so the type of metal oxide constituting the raw material metal oxide particles is not particularly limited. From the viewpoint of usefulness, for example, aluminum oxide (Al ₂ O ₃ , etc.), zirconium oxide (ZrO ₂ , etc.), titanium oxide (TiO _2, etc.), silica (SiO ₂ , etc.), cerium oxide, zinc oxide (ZnO, etc.), vanadium oxide, iron oxide (FeO, Fe ₂ O ₃ , etc.), chromium oxide (Cr ₂ O ₃ , Cr ₃ O ₄ , etc.), molybdenum oxide (MoO ₃ , etc.), niobium oxide (Nb ₂ O ₅ , etc.), tantalum oxide, tungsten oxide (WO ₃ , WO ₂ , etc.), manganese oxide, haumium oxide, etc. are commonly used as metal oxides. The metal oxide may be a composite metal oxide, for example, ilmenite (FeTiO ₃ ) contained in lunar soil. The particles formed of these metal oxides may be used alone or in combination of two or more kinds.

Of these metal oxide particles, the reduction device of the present invention can uniformly reduce even difficult-to-reduc metal oxide particles, and is therefore suitable for use with difficult-to-reduc metal particles such as zirconium oxide, titanium oxide, niobium oxide, chromium oxide, and tungsten oxide. Furthermore, since it produces water and oxygen, it can also be used to reduce metal oxide particles on the lunar surface, and therefore can also be used appropriately with iron oxide and ilmenite.

The average particle size of the metal oxide particles can be appropriately selected depending on the reaction conditions and is not particularly limited, but from the viewpoint of the production efficiency of the reduced product, it is, for example, 1 to 1000 μm, preferably 2 to 100 μm, further preferably 3 to 70 μm, more preferably 5 to 50 μm, and most preferably 10 to 30 μm. If the particle size is too small, there is a risk that the handling properties will decrease, and if it is too large, there is a risk that it will be difficult to reduce uniformly.

In this specification and claims, the average particle size of metal oxide particles can be measured by conventional methods, for example, it means the median particle size (D50) measured on a volume basis using a laser diffraction particle size distribution analyzer.

The supply amount of metal oxide particles is, for example, 1.0 to 58 g/min, preferably 1.0 to 20 g/min, more preferably 2.0 to 10 g/min, and even more preferably 2.5 to 5.8 g/min. If the supply amount is too small, the productivity of the reduced product may decrease, and if the supply amount is too large, it may be difficult to reduce uniformly.

(reducing gas)
The reducing gas may contain hydrogen gas, and may be hydrogen gas alone or a mixture of hydrogen gas and an inert gas. When a mixture of hydrogen gas and an inert gas is used as the reducing gas, the degree of reduction of the metal oxide particles can be easily adjusted by adjusting the hydrogen gas concentration in the reducing gas. Examples of the inert gas include nitrogen gas, helium gas, and argon gas.

The concentration of hydrogen gas in the reducing gas can be selected from the range of 0.5 to 100% by volume, preferably 1 to 80% by volume, more preferably 1.5 to 50% by volume, even more preferably 2 to 30% by volume, and most preferably 3 to 10% by volume. If the hydrogen gas concentration is too low, there is a risk that the productivity of the reduced material will decrease.

The flow rate of the reducing gas is, for example, 1 to 100 liters/minute, preferably 2 to 50 liters/minute, more preferably 3 to 30 liters/minute, even more preferably 5 to 20 liters/minute, and most preferably 8 to 15 liters/minute. If the flow rate is too small, the productivity of the reduced material may decrease, and if the flow rate is too large, it may become difficult to adjust the degree of oxygen deficiency.

In this specification and claims, the hydrogen gas concentration in the reducing gas and the flow rate of the reducing gas refer to the volume ratio at atmospheric pressure and 0°C.

[Reduced form]
In the reduction device of the present invention, the metal oxide particles can be easily reduced with a high conversion rate, and the oxygen deficiency of the resulting reduced product can be easily controlled by adjusting reaction conditions such as the hydrogen gas concentration in the reduction gas, the flow rate of the reduction gas, the temperature, the pressure, the stirring speed, and the reduction time. Therefore, in the method for producing a reduced product using the reduction device of the present invention, both metal particles and oxygen-deficient metal oxide particles can be produced as the reduced product. In addition, even if the reduced product is a difficult-to-reduc metal oxide particle such as oxygen-deficient tungsten oxide particles, a uniform reduced product can be easily produced. Furthermore, in the production method of the present invention, the reduced product can be continuously produced, and not only can the desired oxygen deficiency be easily adjusted by changing the flow conditions of the reduction gas and the shape of the stirring and moving means, but the process can also be easily scaled up by enlarging the shape of the reactor that houses the stirring and moving means. In particular, the conversion rate of the resulting reduced product is about 100%, so that the desired reduced product can be obtained efficiently.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
In the reduction apparatus shown in FIG. 1, the metal oxide particles were reduced to produce a reduced material without supplying the produced exhaust gas to a condensation unit (without recycling hydrogen gas).

Specifically, tungsten oxide _WO3 particles (manufactured by A.L.M.T.L. Co., Ltd., average particle size 20 μm) were used as the raw material metal oxide particles and were supplied to the reactor at a rate of 2.5 g/min to 5.8 g/min., and a mixed gas of hydrogen gas and argon gas with a hydrogen gas concentration of 3 vol% (0° C.) was used as the reducing gas, and the flow rate of the mixed gas was adjusted to 10 L/min.

The reactor used was a cylindrical reactor (NCF600 tube) with an inner diameter of 32.9 mm and an outer diameter of 50 mm, in which a screw (outer diameter 31 mm, shaft outer diameter 7 mm, pitch 38 mm, blade inclination angle 65°, blade thickness 6.5 mm, S55C screw) was housed inside, and an electric furnace ("ARF1110-300" manufactured by Asahi Rika Seisakusho Co., Ltd.) was used as the heater. The reaction conditions were as follows: the screw was rotated at a rotation speed of 0.3 to 1.1 rpm, the reaction tube inlet pressure was fixed at 300 kPa, the reduction temperature was changed in the range of 1053 to 1123 K, and the reduction time was changed in the range of 7 to 26 minutes, and the effect of each operating condition on the reduction reaction was examined.

The molar fraction of the reduced product obtained under the condition of a reduction temperature of 1073K was calculated from the XRD analysis results and is shown in Figure 3. As is clear from the results of Figure 3, when the reduction time was 7 minutes, mainly _WO3 , _WO2.9 , and _WO2.72 were generated, and when the reduction time was 26 minutes, the generation of _WO2 and W was also confirmed. In other words, it is possible to control the reduction degree by manipulating the reduction time, and when the reduction time was 10 minutes, the molar fraction of oxygen-deficient _WO2.72 reached 0.95. In addition, the reduction time was inversely proportional to the screw rotation speed, and when the reaction time was short, the stirring effect of the screw in the reaction tube was enhanced, making it possible to perform a more uniform reduction.

The XRD analysis was performed using an X-ray diffraction device ("SmartLab" manufactured by Rigaku Corporation).

FIG. 4 shows a cross-sectional SEM image of the raw material WO ₃ particles, and FIG. 5 shows a cross-sectional SEM image of the reduced product with a reduction temperature of 1123K and a reduction time of 7 minutes. The molar fraction after reduction was WO _2.72 : 0.85, WO ₃ : 0.14 according to XRD. The cross section of the raw material particle showed cracks inside the particle, but no fine gaps were observed throughout the particle. On the other hand, fibrous crystal growth was confirmed throughout the reduced product after reduction. It has been reported in Y. Sun, et al., Electrochim. Acta, 187, 329-339 (2016) that WO _2.72 exhibits anisotropic growth behavior, and this result is consistent with this behavior. That is, it was confirmed that the main component after a reduction time of 7 minutes was WO _2.72 , and the reaction had progressed to the inside of the particle.

The reduction device of the present invention can be used to reduce metal oxides, and the resulting reduced products, oxygen-deficient metal oxides, can be used as various semiconductor materials, such as photocatalysts and negative electrode materials for lithium-ion batteries. In addition, the reduction device of the present invention can also produce water as a by-product in addition to the reduced products, and can therefore be used as a plant that generates water using lunar soil.

1... Reduction device 2... Metal oxide particles 3... Feeder (hopper)
4... Reactor 4a... Supply port 4b... Outlet port 4c... Inlet port 4d... Discharge port 5... Mobile stirring means (screw)
6: Driving device 7: Heater 8: Collection container

Claims (12)

A reduction apparatus including a reaction unit having a reactor for reducing metal oxide particles with hydrogen gas and a circulation means for circulating a reduction gas containing the hydrogen gas through the reactor,
the reactor has a supply port for supplying the metal oxide particles;
an outlet for recovering a reduced product obtained by reducing the metal oxide particles;
a stirring and moving means for stirring and moving the metal oxide particles supplied from the supply port to the outlet in the presence of the reducing gas;
an inlet for introducing the reducing gas;
and an outlet for recovering exhaust gas after the reaction.
Furthermore, the reactor has a cylindrical space as a reaction chamber, and the stirring and moving means is a screw housed in the cylindrical space,
The outer diameter of the screw is 90% or more of the inner diameter of the reactor,
The reduction device, wherein the circulating means is a circulating means for circulating the reducing gas in a direction opposite to a moving direction of the metal oxide particles. 2. The reduction apparatus according to claim 1 , wherein the outer diameter of the screw is 92 to 98% of the inner diameter of the reactor. 3. The reduction apparatus according to claim 1, wherein the reaction unit further comprises a heating means for heating the reactor. 4. The reduction apparatus according to claim 3 , wherein the inlet is formed downstream of a region heated by the heating means in a moving direction of the metal oxide particles. 5. The reduction apparatus according to claim 1, wherein the reducing gas is hydrogen gas alone or a mixed gas of hydrogen gas and an inert gas. The reduction apparatus according to any one of claims 1 to 5 , wherein the metal oxide particles are reduced at a temperature of 853 to 1323 K and a pressure of 150 to 500 kPa. The reduction device according to any one of claims 1 to 6 , wherein the metal oxide particles have an average particle size of 1 to 1000 µm. The reduction device according to any one of claims 1 to 7 , wherein the reductant is an oxygen-deficient metal oxide particle. The reduction apparatus according to any one of claims 1 to 8, further comprising a circulation line for circulating the reducing gas contained in the exhaust gas to the reactor. The reduction device according to any one of claims 1 to 9 , further comprising an electrolysis unit that electrolyzes water contained in the exhaust gas into hydrogen gas and oxygen gas, and circulates the hydrogen gas generated in the electrolysis unit to the reactor. 10. A method for continuously producing a reduced material of a metal oxide, using the reduction apparatus according to any one of claims 1 to 9 , comprising moving the metal oxide particles supplied from the supply port to the outlet while stirring them, while circulating the reducing gas in a direction opposite to a direction of movement of the metal oxide particles, and recovering the reduced material from the outlet. 10. A method for controlling a degree of oxygen deficiency in a metal oxide by adjusting at least one condition selected from the group consisting of a hydrogen gas concentration in the reducing gas, a flow rate of the reducing gas, a temperature, a pressure, a stirring speed, and a reduction time, the method comprising: moving the metal oxide particles supplied from the supply port to the outlet while stirring the metal oxide particles in the supply port and recovering the oxygen-deficient metal oxide particles , which are the reduced product, from the outlet, using the reduction apparatus according to any one of claims 1 to 9.

Images