JP2776182B2 - Method for producing hydrogen storage alloy powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hydrogen storage alloy powder used for a negative electrode of a Ni-hydrogen secondary battery, a hydrogen storage / transport device, a heat pump, a hydrogen purifier, a heat-driven switchgear and the like. More specifically, the present invention relates to a method for producing a hydrogen storage alloy powder having a reduced oxygen content by a gas atomization method.

[0002]

2. Description of the Related Art Until now, applications of a hydrogen storage alloy have been proposed for a negative electrode of a Ni-hydrogen secondary battery, a hydrogen storage / transport device, a heat pump, a hydrogen purification device, a thermally driven switching device, and the like.

[0003] Among these applications, a non-polluting, high-capacity Ni-hydrogen battery has been developed as a replacement battery for Ni-Cd batteries used in memory backup of AV equipment and notebook computers, and mobile cellular phones. The use of hydrogen storage alloys in batteries has recently been expanding. The hydrogen storage alloy is used in a powder form in many applications, including when used as a negative electrode active material of a Ni-hydrogen battery.

For the purpose of improving the performance of the hydrogen storage alloy, a method of preventing segregation by rapidly cooling during solidification of the hydrogen storage alloy to make the alloy structure uniform and fine has been attempted in recent years. As one of the methods, there is production of a hydrogen storage alloy powder by a gas atomizing method. According to the gas atomizing method, not only can the alloy structure be homogenized and refined by rapid solidification, but also the hydrogen storage alloy can be obtained in powder form, so that the pulverizing step can be omitted or reduced.

The production of a hydrogen storage alloy by the gas atomization method is generally performed in a batch system by an apparatus schematically shown in FIG. A non-oxidizing gas such as Ar is introduced into the atomizing tank to make the inside a non-oxidizing atmosphere. afterwards,
The molten raw material adjusted to have a predetermined alloy composition is melted in a melting furnace in a melting chamber in a vacuum or a non-oxidizing atmosphere such as Ar, and the obtained molten metal is poured into a tundish, and the molten metal is poured into a tundish. Flow down the atomizer tank little by little from the melt nozzle at the bottom of the dish. A high-speed non-oxidizing gas such as Ar is sprayed into the molten metal stream in an atomizing tank to form powder, and alloy powder cooled and solidified while falling in the tank is provided at the bottom of the atomizing tank as illustrated. It is discharged into a powder recovery container and / or separated from the atomized gas by a cyclone separator connected to an atomizing tank, discharged into a powder recovery container and deposited in this container. After the completion of one batch of processing (usually after the whole amount of the dissolved raw material has been pulverized through a tundish),
The alloy powder deposited in the powder recovery container is recovered at one time.

[0006]

One of the important factors influencing the performance of a hydrogen storage alloy is the oxygen content of the alloy. The lower the oxygen content, the better the performance of the alloy. In the conventional gas atomization method, in order to reduce the alloy oxygen content,
Non-oxidizing gas (eg, Ar gas) constituting the atmosphere of the melting chamber containing the melting furnace and the tundish and the atmosphere of the gas atomizing tank where the molten metal is powdered is made as low oxygen as possible. However, this is greatly affected by the purity of the non-oxidizing gas used and the confidentiality of the dissolution chamber and the atomizing tank, and thus has an industrial limit.

[0007] Under such circumstances, the present invention aims at further reducing the oxygen content of the hydrogen storage alloy powder by a method which can be easily realized industrially. Specifically, an object of the present invention is to provide a method for producing a hydrogen storage alloy powder by a gas atomization method, in which oxidation in a cooling process of the powder in an atomizing tank and a powder recovery container is suppressed as much as possible.

[0008]

The present inventors have found that the main cause of the increase in the amount of oxygen in the hydrogen storage alloy powder produced by the gas atomization method is that the powder that has fallen in the atomization tank and accumulated in the powder recovery container has Focusing on the fact that the alloy powder that is still at high temperature undergoes high-temperature oxidation due to trace amounts of oxygen in the atmosphere during the cooling process, forming an oxide film on the powder surface, and suppressing this high-temperature oxidation as much as possible In order to obtain an alloy powder having a reduced oxygen content, studies were advanced.

As a result, after the molten metal is pulverized by gas spraying, the solidified powder is cooled at an increased cooling rate by accelerated cooling, so that the temperature of the alloy powder being deposited in the powder recovery container exceeds a certain temperature. It has been found that if it is not set, the formation of an oxide film on the powder surface, which occurs at a high temperature during the cooling process, can be effectively suppressed.

Here, the present invention is directed to a batch method in which a high-speed gas is sprayed into a molten metal stream, and the generated powder is cooled and solidified, deposited in a powder recovery container, and after one batch of processing is completed, the powder is recovered from the container. In the production of the hydrogen storage alloy powder by the gas atomization method, the maximum powder temperature at the time when one batch of powder has been deposited in the container is 400 ° C. or less,
A gist of the present invention is a method for producing a hydrogen storage alloy powder, which is characterized in that the produced powder is acceleratedly cooled.

[0011]

In order to reduce the amount of oxygen in the hydrogen storage alloy powder produced by the gas atomizing method, it is general to reduce the amount of oxygen in the atmosphere gas and the atomized gas in the melting chamber and the atomizing tank. However, this method has limitations for industrial implementation, and when the obtained low-oxygen powder is used for applications such as a negative electrode of a Ni-hydrogen secondary battery, sufficient performance with respect to the initial activity and capacity is required. It turned out that it could not be obtained.

Therefore, as a result of various studies of the cause, the cooling rate of the alloy powder generated by gas spraying of the molten metal has an influence on the formation of the oxide film on the alloy surface,
Specifically, accelerated cooling so that the alloy powder temperature at the end of deposition in the powder recovery container does not exceed 400 ° C at the maximum,
It was found that generation of surface oxides could be suppressed.

The reason is presumed as follows at present. The maximum powder temperature in the powder collection container is 40 at the end of deposition
If the temperature exceeds 0 ° C, the cooling rate after solidification of the powder is low, and the rare earth elements (La,
Oxidizable elements such as Ce, Pr) and Mn, Al, Ti, Zr and V undergo oxidation during cooling, and a strong oxide film is formed on the powder surface. For this reason, the surface coating serves as a barrier, and it becomes difficult for hydrogen to be absorbed, so that the performance of the hydrogen storage alloy is reduced. When the powder temperature is maintained at 400 ° C. or lower, even if the element forms an oxide film, the oxide film is not strong and hydrogen can easily pass through the film, and thus does not become a barrier. The temperature of the powder deposited in the powder recovery container generally increases as the amount of deposition increases because the amount of heat generated by the hot powder is larger than the amount of heat removed in the container. Therefore, the maximum temperature is reached at the end of the deposition. Therefore, if the temperature at this time does not exceed 400 ° C., a high performance hydrogen storage alloy powder with reduced oxide film formation can be obtained.

The method of accelerated cooling so that the maximum temperature of the alloy powder when it has been deposited in the powder recovery container does not exceed 400 ° C. is not limited to a specific cooling method. By forced cooling using gas and / or water,
This accelerated cooling can be achieved.

FIGS. 3 to 5 show examples of a forced cooling method which can be employed to achieve such accelerated cooling. Fig. 3 (a)
(B) shows a cooling method using gas. As the cooling gas, a non-oxidizing gas such as Ar gas is used. In the method shown in FIG. 3 (a), a cooling gas is blown into a cyclone separator for separating powder from gas to forcibly cool the powder in the cyclone. In the method shown in FIG. 3 (b), a cooling gas is blown from the lower part in the atomizing tank, and the powder falling in the tank is forcibly cooled. Cooling gas introduction direction is
When turned upward as shown, the powder is disturbed and cooling efficiency is improved. In each case, the cooling gas is introduced at a flow rate sufficient to achieve the accelerated cooling described above. If gas cooling alone does not provide sufficient cooling, the atomizing tank and / or the powder recovery container may be water-cooled as described below.

FIGS. 4 and 5 show a water-cooling type cooling method, and the hatched portions in the figures are water-cooled portions. Fig. 4 (a) and
As shown in (b), all or part of the periphery of the atomizing tank, cyclone separator, powder recovery container, and the passage connecting them may be water-cooled, or FIG.
As shown in (b) and (b), it is also possible to install a water-cooled baffle plate in the atomizing tank and perform forced cooling while the powder rolls on this baffle plate.

Of course, the cooling methods shown in these figures may be appropriately combined, and other methods may be used in combination or alone. The cooling rate is not particularly limited, but it is necessary to set the cooling rate so that the temperature of the alloy powder when the powder is completely deposited in the powder collecting container does not exceed 400 ° C. Generally, as the throughput of one batch increases, the powder temperature during deposition tends to increase, so that it is necessary to increase the cooling rate. That is, the cooling rate at which the powder deposition does not exceed 400 ° C. at the end of the powder deposition varies depending on the amount dissolved in one batch and other operating conditions. Therefore, if necessary, experiments should be performed to determine the cooling method and cooling rate. I just need. Accordingly
Therefore, “accelerated cooling” referred to in this specification means that
The alloy powder temperature at the end of loading will not exceed 400 ° C
Cooling at a rapid cooling rate
Or it is synonymous with forced cooling.

FIG. 6 shows an atomizing device conventionally used. In this case, the cooling of the powder generated by the gas spraying is the cooling by the atmospheric gas when falling in the atomizing tank together with the spraying gas and passing through the cyclone and the natural cooling during the deposition in the powder recovery container. . The spray gas is
Since the temperature is high due to contact with the molten metal, the cooling capacity is small. Therefore, the temperature of the powder when it has been deposited in the powder recovery container is generally as high as 600 to 800 ° C.
As a result of being exposed to such a high temperature for a long time, an oxide film is formed on the surface of the powder due to oxidation by a trace amount of oxygen existing in the surrounding atmosphere.

The maximum temperature of the alloy powder deposited in the powder recovery container can be measured, for example, by a thermocouple installed in the container as shown in FIG. Since the temperature of the powder in the container varies depending on the position, the position where the temperature becomes the highest may be determined by an experiment, and the thermocouple may be installed at that position. The position at which the temperature becomes highest depends on the cooling method, but is usually an intermediate position in the height direction of the powder that has been deposited.

The method of the present invention can be applied to the production of any hydrogen storage alloy. Representative examples of useful hydrogen storage alloy as a negative electrode material of Ni- MH batteries, AB ₅ type is a type AB or AB ₂ type hydrogen storage alloy. Examples of AB ₅ type alloys, LaNi _x or MmNi _x (x is 4.7 to 5.2) as a basic structure, a portion of the Ni Co, Mn, Al, Fe , Cr, Cu, V, Be, Zr,
They are substituted with one or more elements such as Ti, Mo, W and the like. Since LaNi _x is expensive and has a short life, use of MmNi _x is practically preferable. An example of the AB type alloy is TiNi, but part of Ti may be replaced with Zr and Hf, and part of Ni may be replaced with Co, Mn, Fe, Al, V, Cr and Mo. Examples of AB ₂ type alloys, ZrV _y (y is from 1.9 to 2.25) as a basic structure, a portion of the V Ni, Mn, Cr, Co , Fe, Al, Mo,
It is substituted by one or more elements such as W, Cu, Be.

The hydrogen storage alloy powder produced by the method of the present invention is further pulverized in a non-oxidizing gas atmosphere such as Ar, if necessary, and adjusted to a desired particle size, and then used for applications such as electrode production. Is done.

[0022]

【Example】

(Production of hydrogen storage alloy powder by gas atomization method) Fig. 1
The hydrogen storage alloy powder having the composition shown in Table 1 was produced by the method described below using the gas atomizer shown in Table 1. The accelerated cooling of the alloy powder generated in the atomizing tank was performed by the cooling method shown in FIG. Ar gas used as a non-oxidizing gas has the purity that is normally used for industrial use.
It had a content of 99.99% and an oxygen content of 1 ppm.

[0023]

[Table 1]

Each of the dissolved materials used had a high purity of 99% or more. The molten raw materials were mixed so as to produce a hydrogen storage alloy having a predetermined composition, and were subjected to high frequency melting in a melting furnace in a melting chamber maintained in an Ar gas atmosphere. The amount of one dissolution was 70 kg. Then, the whole amount of this molten metal,
It was made to flow down through a tundish in the melting chamber, and was pulverized by spraying Ar gas on the melt flow just below the nozzle. The gas atomization conditions are the spray pressure 5
The weight was 0 kgf / cm ² and the melt diameter was 6 mm. The time required for flowing the molten metal was about 2 minutes.

The powder generated in the tank was deposited in a powder recovery container. By changing the flow rate of the cooling water in the baffle plate, the maximum powder temperature in the powder recovery container when the powder was completely deposited in the recovery container was changed as shown in Table 2.
This temperature is a temperature measured by a thermocouple installed at an intermediate position of the powder height in the collection container. Note that the cooling condition 1 is a case where cooling was not performed at all.

[0026]

[Table 2]

(Evaluation of performance as negative electrode for Ni-hydrogen battery) The hydrogen-absorbing alloy powder obtained above was ball-milled in an Ar gas atmosphere, and -100 µm powder was collected by sieving. Using this alloy powder as a negative electrode material, a Ni-hydrogen battery was constructed, and its discharge capacity was measured. The outline of the test method is shown below.

(1) Configuration of Electrode A 100 μm alloy powder was made into a paste with an aqueous solution of polyvinyl alcohol in an amount corresponding to 5% of the weight of the powder, filled in a porous nickel foam, dried, and then subjected to vacuum hot pressing. To form a hydrogen storage alloy electrode. The filling amount of the hydrogen storage alloy per electrode was 2 g.

(2) Configuration of Battery The hydrogen storage alloy electrode manufactured above was used for the negative electrode, and a commercially available nominal 2000 mA sintered Ni electrode was used for the positive electrode. A 6M-KOH aqueous solution was injected as an electrolytic solution with a polyamide nonwoven fabric interposed therebetween, followed by sealing, thereby producing a sealed Ni-hydrogen battery for testing. This battery was a negative-electrode capacity-regulated battery in which the capacity of the positive electrode was sufficiently larger than the capacity of the negative electrode.

(3) Evaluation of battery performance By repeating charging at 50 mA / g (25 ° C.) for 8 hours and discharging at 50 mA / g (25 ° C.) to a terminal voltage of 0.9 V, each discharge cycle is repeated. The discharge capacity was measured. The discharge capacity thus measured was defined as the discharge capacity of the hydrogen storage alloy used for the negative electrode. The charge / discharge was repeated, and the discharge capacity measured at the tenth cycle is shown in Table 3.

[0031]

[Table 3]

(4) Results The results in Table 3 are graphed in FIG. FIG. 2 clearly shows the effect of the temperature of the hydrogen storage alloy powder in the recovery container on the discharge capacity at the end of the deposition. That is, the discharge capacity at the tenth cycle becomes larger as the powder temperature is lower, but becomes almost constant when the powder temperature is 400 ° C. or lower. From this result, according to the present invention, when accelerated cooling so that the powder temperature in the collection container at the end of the deposition becomes 400 ° C. or less, a decrease in the discharge capacity due to oxidation is avoided, and hydrogen having a larger discharge capacity than the conventional product is obtained. It can be seen that the occlusion alloy powder is obtained.

[0033]

As described in detail above, if the produced powder is accelerated and cooled at the time of gas atomization by the method of the present invention and the powder in the collection container is kept at 400 ° C. or lower, the powder surface which is likely to occur at a high temperature is obtained. The formation of a strong oxide film at the same time can be suppressed, and a high-performance hydrogen storage alloy with a small amount of oxygen can be manufactured. As a result, the resulting powder
When used for a negative electrode for a Ni-hydrogen battery or the like, there is little adverse effect due to the oxide film, and excellent performance such as initial activation ability and capacity can be exhibited.

[Brief description of the drawings]

FIG. 1 is a schematic view of a gas atomizing apparatus for producing an alloy powder.

FIG. 2 is a graph showing the relationship between the temperature of a hydrogen storage alloy powder in a recovery container at the end of deposition and the discharge capacity at the 10th cycle of a Ni-hydrogen battery using this powder as a negative electrode active material.

FIG. 3 is an explanatory view showing a method of cooling atomized powder by gas cooling.

FIG. 4 is an explanatory view showing a method of cooling atomized powder by water cooling.

FIG. 5 is an explanatory view showing another cooling method of atomized powder by water cooling.

FIG. 6 is an explanatory view showing a conventional atomizing device.

FIG. 7 is an explanatory diagram showing a method for measuring the temperature of the powder in the collection container.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideya Kaminaka 4-5-33 Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries, Ltd. (72) Inventor Sumio Toyosumi 4-5-33 Kitahama, Chuo-ku, Osaka City (56) References JP-A-5-1305 (JP, A) JP-A-58-22310 (JP, A) JP-A 5-63207 (JP, A) (58) Fields investigated (Int.Cl. ⁶ , DB name) B22F 9/08 B22F 1/00 C22C 19/00

Claims (1)

(57) [Claims] 1. Hydrogen storage by a batch gas atomization method in which a high-speed gas is sprayed into a molten metal stream, and the generated powder is cooled and solidified, deposited in a powder recovery container, and recovered after the completion of one batch processing. In the production of alloy powder, the production powder is accelerated and cooled so that the maximum powder temperature at the time when one batch of powder has been deposited in the container is 400 ° C. or less, the production of hydrogen storage alloy powder being characterized in that Method.