Method for manufacturing metal powder

Abstract

A method for manufacturing metal powder includes: melting at least a portion of a metal starting material in a reaction vessel by utilizing plasma so as to form molten metal; evaporating the molten metal so as to produce a metal vapor; and transferring the metal vapor from the reaction vessel to a cooling tube together with a carrier gas supplied into the reaction vessel so as to cool the metal vapor, and condensing the metal vapor in the cooling tube, thereby producing metal powder. The method further includes supplying an oxygen gas into the reaction vessel.

Claims

1. A method for manufacturing metal powder comprising: melting at least a portion of a metal starting material in a reaction vessel by utilizing plasma so as to form molten metal; evaporating the molten metal so as to produce a metal vapor; transferring the metal vapor from the reaction vessel to a cooling tube together with a carrier gas supplied into the reaction vessel so as to cool the metal vapor; and condensing the metal vapor in the cooling tube, thereby producing metal powder, wherein the method further comprises supplying an oxygen gas into the reaction vessel, during melting, and at least a part of the reaction vessel is formed of an oxide ceramic material, the part contacting the molten metal.

2. The method for manufacturing metal powder according to claim 1, wherein the oxide ceramic material is zirconia-based ceramic.

3. The method for manufacturing metal powder according to claim 1, wherein the oxygen gas is supplied at an amount of 1500 mL/min or less for a metal powder production amount of 1 Kg/hr.

4. The method for manufacturing metal powder according to claim 1 further comprising supplying an additional element selected from sulfur, phosphorus, platinum, rhenium, zinc, tin, aluminum and boron into the reaction vessel, during melting.

5. The method for manufacturing metal powder according to claim 4, wherein the additional element is supplied in a form of an organic compound and/or a hydrogen compound.

6. The method for manufacturing metal powder according to claim 1, wherein the metal powder contains 50% by weight or more of a base metal and the base metal is nickel, copper, cobalt, iron, tantalum, titanium or tungsten.

7. The method for manufacturing metal powder according to claim 1, wherein the plasma is transferred DC arc plasma.

8. The method for manufacturing metal powder according to claim 1, wherein the oxygen gas is supplied at an amount of 0.05 mL/min or more for a metal powder production amount of 1 Kg/hr.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows a plasma device used in Examples.

(2) FIG. 2 shows a plasma device used in a conventional example.

EMBODIMENT FOR CARRYING OUT THE INVENTION

(3) Metal powder manufactured by a method for manufacturing metal powder of the present invention is exemplified by but not limited to: precious metals such as silver, gold, and platinum group metals; base metals such as nickel, copper, cobalt, iron, tantalum, titanium, and tungsten; and alloys containing any of these. It is particularly preferable that the metal powder be metal powder containing a base metal as a main component so that the effects of the present invention can be enjoyed more.

(4) The main component herein means that a percentage of a base metal in the entire metal powder is 50 weight % or more.

(5) In the method for manufacturing metal powder of the present invention, a metal starting material is not particularly limited as long as it is a substance containing a metal component of target metal powder, and usable examples include, other than a pure metal, an alloy, a composite, a mixture and a compound each containing two or more types of metal components. Although there is no special limitation, it is preferable, in terms of easy handling, to use a granular or massive metal material or alloy material having a size of about several mm to several ten mm.

(6) Hereinafter, a process of the present invention is described with an example.

(7) A metal as a staring material is supplied from a starting-material feed port into a reaction vessel of a plasma device.

(8) Into the reaction vessel, oxygen and a dilute gas, which is not essential, are supplied. The metal starting material is melted by plasma in the reaction vessel and accumulated at a crucible part, which is the lower part of the reaction vessel, as molten metal. The molten metal is further heated by the plasma to evaporate, so that a metal vapor is produced. The produced metal vapor is transferred from the reaction vessel to a cooling tube by a carrier gas containing a plasma gas used for producing the plasma and the dilute gas supplied as needed, and cooled and condensed in the cooling tube. Thus, metal powder is produced.

(9) Material which constitutes the reaction vessel is not limited, and a refractory material conventionally used for plasma devices, such as graphite or ceramic, is used therefor. In particular, when at least the crucible part is made of an oxide ceramic material, zirconia-based ceramic in particular, the effects of the present invention are remarkable.

(10) As the plasma gas and the dilute gas, an inert gas or a reducing gas usually used in manufacturing metal powder is used. Examples thereof include argon, helium, nitrogen, ammonia, methane, and a mixture of any of these.

(11) The oxygen gas maybe supplied as a gas containing oxygen, such as air or a mixed gas of an inert gas and oxygen, instead of a pure oxygen gas. The oxygen may be mixed with the dilute gas and supplied into the reaction vessel, or may be unmixed with the dilute gas and supplied into the reaction vessel from an introduction port which is different from that for the dilute gas.

(12) Although the reason why the amount of impurities is reduced by supply of an oxygen gas into a reaction vessel is not completely clear, it may be considered as described below with a case taken as an example, the case where nickel powder is manufactured using metal nickel as a metal starting material and using a reaction vessel made of stabilized zirconia (hereinafter, may be referred to as a zirconia crucible indicating the crucible part).

(13) In a conventional method, at a solid-liquid interface where the zirconia crucible and high-temperature molten nickel come into contact with each other, oxygen inside the crucible moves into the molten nickel, and metals produced thereby, such as zirconium, calcium and yttrium, dissolve in the molten nickel, so that impurities in the nickel powder to be produced increase. Because zirconia has a property as a solid electrolyte at a high temperature, 1000 C. or more in particular, and has high ion conductivity, the eluted-and-dissolved amount of the oxygen and the metals becomes large by the oxygen moving from the inside of the crucible to the solid-liquid interface. In the present invention, however, it is assumed that oxygen introduced into the reaction vessel dissolves in the molten nickel, and an oxygen concentration in the molten nickel becomes high, so that the oxygen from the crucible is kept from moving, and the amount of impurities derived from the crucible in the produced nickel powder reduces.

(14) Regarding an oxygen gas supply, even with a small amount of about 0.05 mL/min as the supply for a metal powder production rate of 1 Kg/hr, the effect of reducing impurities is observed.

(15) In the present invention, an oxygen supply which is necessary to obtain the effect of reducing impurities equivalent to the above is approximately proportional to a supply rate of a metal starting material (metal powder production rate). Hence, hereinafter, the oxygen supply is expressed as an amount for a metal powder production rate of 1 Kg/hr. The oxygen gas supply is expressed as a flow rate of an oxygen gas at 25 C. and 1 atm. It is particularly preferable that oxygen be supplied at an amount of 0.1 mL/min or more so that the remarkable effects are obtained.

(16) On the other hand, when the oxygen gas supply is large, problems arise. For example, the manufacturing efficiency decreases because too much oxygen dissolves in molten metal and the surface of the molten metal is oxidized or plasma becomes unstable; a heat insulating material or the like used for the reaction vessel is burned; and, in DC plasma, an electrode metal is oxidized. Further, of the supplied oxygen, oxygen which has not been consumed either to keep the crucible components from being eluted, which is described above, or to decompose compounds, which is described below, constitutes a portion of a carrier gas. Hence, it is necessary to adjust the oxygen gas supply to an amount with which oxidation does not occur when the metal vapor is condensed in the cooling tube and thereby metal powder is precipitated. Although it differs depending on the type of a target metal and additional elements described below, it is preferable that the oxygen gas supply not exceed a maximum of 1500 mL/min in the case where there is no additional element described below. It is particularly preferable that an oxygen gas be supplied at an amount of 0.1 to 1000 mL/min so that the above problems hardly occur and the remarkable effects are obtained.

(17) As described above, impurities tend to increase when, in order to make metal powder contain an element (s) such as sulfur, phosphorus, platinum, rhenium, zinc, tin, aluminum and boron as an additional element(s), compounds of these additional elements, particularly organic compounds, hydrogen compounds or the like, are supplied into the plasma reaction vessel. In this case, supply of oxygen is preferable because the effect of reducing impurities thereby is particularly remarkable and the effects of the present invention can be enjoyed more. That is, although it is assumed that elution of oxygen from the crucible and dissolution thereof in molten metal, which is described above, more easily occur because the above organic compounds or hydrogen compounds decompose in a high-temperature gaseous phase and show reducibility, supply of oxygen cancels out the reducibility and is extremely effective in reducing impurities.

(18) It is also considered that oxygen has an effect of promoting decomposition of these compounds so as to make it easy for metal powder to contain an additional element(s). Hence, it is preferable that oxygen be supplied more than a stoichiometric amount necessary for decomposition of the above organic compounds or hydrogen compounds.

(19) Usable examples of the above organic compounds include but are not limited to: in the case of sulfur, thiols such as methanethiol and ethanethiol; mercaptan compounds such as mercaptoethanol and mercaptobutanol; thiophenes such as benzothiophene; and thiazoles.

(20) In the case of phosphorus, usable examples thereof include: phosphines such as triphenylphosphine, methylphenylphosphine and trimethylphosphine; and phosphorane.

(21) In the case of platinum, rhenium, zinc, tin, aluminum and boron, examples of the organic compounds include: carboxylates; amine complexes; phosphine complexes; mercaptides; and organic derivatives of rhenic acid.

(22) Usable examples of the above hydrogen compounds include: hydrides such as hydrogen sulfide, aluminum hydride, and diborane; and organic derivatives thereof.

(23) Further, in the present invention, it is preferable that the above plasma be transferred DC arc plasma so that the effects of the present invention can be enjoyed more.

EXAMPLES

(24) Next, the present invention is detailed with Examples. However, the present invention is not limited thereto. In Examples below, a flow rate of each gas is expressed by a flow rate of a gas at 25 C. and 1 atm, as with oxygen.

(25) In Examples described below, a transferred DC arc plasma device 1 shown in FIG. 1 was used as a plasma device.

(26) As a reaction vessel 2 of the device, a reaction vessel made of calcium stabilized zirconia is used. At the upper part of the reaction vessel 2, a plasma torch 4 is placed, and a plasma producing gas is supplied to the plasma torch 4 through a not-shown supply tube. The plasma torch 4 produces plasma 7 with a cathode 6 as the negative pole and a not-shown anode provided inside the plasma torch 4 as the positive pole, and after that, the positive pole is transferred to an anode 5, so that the plasma 7 is produced between the cathode 6 and the anode 5. At least a portion of a metal starting material which is supplied from a not-shown starting-material feed port to a crucible part 9 of the reaction vessel 2 is melted by heat of the plasma 7, so that molten metal 8 of the metal is produced. In addition, a portion of the molten metal 8 is evaporated by heat of the plasma 7, so that a metal vapor is produced.

(27) Into the reaction vessel 2, a dilute gas is supplied from a dilute gas supply unit 10. The dilute gas is used as a carrier gas together with the plasma producing gas for carrying the metal vapor to a cooling tube 3. Oxygen is supplied thereinto by introducing air from an oxygen supply unit 11 which is different from the dilute gas supply unit 10.

(28) The metal vapor produced in the reaction vessel 2 is transferred to the cooling tube 3 by the carrier gas containing the plasma producing gas and the dilute gas, and cooled and condensed in the cooling tube 3. Thus, metal powder is produced.

First Example

(29) Into the reaction vessel of the plasma device, a metal nickel mass was supplied as a metal starting material at a supply rate of about 3.0 to 4.0 Kg/hr, argon as a plasma producing gas and a nitrogen gas as a dilute gas were supplied at a flow rate of 70 L/min and a flow rate of 630 to 650 L/min, respectively, and air was supplied at a flow rate with which an oxygen amount became each of those shown in TABLE 1. The device was operated for 500 hours under a condition of plasma output of about 100 kW. Thus, nickel powder was manufactured.

(30) A nickel powder production rate (supply rate of the metal nickel mass); an oxygen supply into the reaction vessel; and a specific surface area, Ca and Zr contents as impurities, and an oxygen content of the obtained nickel powder are all shown in TABLE 1.

(31) The specific surface area of the powder was measured by BET, the Ca and Zr contents were measured with a fluorescence X-ray spectrometer (ZSX100e, manufactured by Rigaku Corporation), and the oxygen content was measured with an oxygen/nitrogen analyzer (EMGA-920, manufactured by Horiba, Ltd.).

(32) TABLE-US-00001 TABLE 1 NICKEL OXYGEN SUPPLY NICKEL POWDER CHARACTERISTICS POWDER (mL/min) FOR SPECIFIC AMOUNT OF PRODUCTION OXYGEN NICKEL POWDER SURFACE IMPURITIES OXYGEN TEST RATE SUPPLY PRODUCTION AREA Ca Zr CONTENT No. (Kg/hr) (mL/min) RATE OF 1 kg/h (m.sup.2/g) (ppm) (ppm) (weight %) 1 4.0 0 0 3.78 123 128 1.21 2 3.9 0.4 0.1 3.96 104 68 1.19 3 3.6 3.6 1.0 3.81 71 28 1.14 4 3.4 34 10 3.56 63 29 0.99 5 3.7 370 100 3.88 50 27 1.16 6 4.0 4000 1000 3.66 45 28 1.10 7 3.2 4800 1500 3.80 83 35 2.10 8 2.4 4800 2000 3.81 108 70 3.03

(33) As it is clear from the result shown in TABLE 1, when the oxygen gas was supplied into the reaction vessel, the amount of impurities was reduced as compared with when no oxygen gas was supplied thereinto (Test No. 1).

(34) In Test No. 8 in which the oxygen supply exceeded 1500 mL/min, although the effect of reducing the amount of impurities was observed, the plasma became unstable. As a result of reducing the supply of the metal nickel in order to maintain the plasma output, the manufacturing efficiency decreased, and also the particle shape and the particle size of the produced nickel powder varied widely.

Second Example

(35) Nickel powder was manufactured in much the same way as First Example, except that a hydrogen sulfide (H.sub.2S) gas was supplied at a rate of 350 mL/min (0.041 mol/min) together with air from the oxygen supply unit 11 into the reaction vessel in order to dope the nickel powder with sulfur.

(36) A nickel powder production rate (supply rate of the metal nickel mass); an oxygen supply into the reaction vessel; and a specific surface area, Ca and Zr contents as impurities, and oxygen and sulfur contents of the obtained nickel powder are shown in TABLE 2. The sulfur content was measured with a carbon/sulfur analyzer (EMIA-320V, manufactured by Horiba, Ltd.).

(37) TABLE-US-00002 TABLE 2 NICKEL OXYGEN SUPPLY NICKEL POWDER CHARACTERISTICS POWDER (mL/min) FOR SPECIFIC AMOUNT OF PRODUCTION OXYGEN NICKEL POWDER SURFACE IMPURITIES OXYGEN SULFUR TEST RATE SUPPLY PRODUCTION AREA Ca Zr CONTENT CONTENT No. (Kg/hr) (mL/min) RATE OF 1 kg/h (m.sup.2/g) (ppm) (ppm) (weight %) (ppm) 9 4.0 0 0 4.6 150 156 1.38 1103 10 3.6 0.4 0.1 4.5 118 77 1.40 1110 11 3.3 3.3 1 4.7 87 34 1.35 1192 12 4.0 200 50 4.6 83 38 1.38 1096 13 3.7 370 100 4.7 60 33 1.43 1154 14 3.1 620 200 5.0 67 40 1.48 1196 15 3.9 3900 1000 4.7 67 35 1.43 1180

(38) As it is clear from the result shown in TABLE 2, when oxygen was supplied into the reaction vessel, the effect of reducing impurities was remarkable.

Third Example

(39) Copper powder was manufactured in the same way as Second Example, except that a metal copper mass was supplied as a metal starting material at a supply rate of about 6.5 to 7.5 Kg/hr into the reaction vessel of the plasma device, and liquid triphenylphosphine was supplied at a rate of 1 mL/min (0.00419 mol/min) together with air from the oxygen supply unit 11 into the reaction vessel in order to dope the copper powder with phosphorus.

(40) A copper powder production rate (supply rate of the metal copper); an oxygen supply into the reaction vessel; and a specific surface area, Ca and Zr contents as impurities, and oxygen and phosphorus contents of the obtained copper powder are shown in TABLE 3. The phosphorus content was measured with a fluorescence X-ray spectrometer (ZSX100e, manufactured by Rigaku Corporation).

(41) TABLE-US-00003 TABLE 3 COPPER OXYGEN SUPPLY COPPER POWDER CHARACTERISTICS POWDER (mL/min) FOR SPECIFIC AMOUNT OF PRODUCTION OXYGEN COPPER POWDER SURFACE IMPURITIES OXYGEN PHOSPHORUS TEST RATE SUPPLY PRODUCTION AREA Ca Zr CONTENT CONTENT No. (Kg/hr) (mL/min) RATE OF 1 kg/h (m.sup.2/g) (ppm) (ppm) (weight %) (ppm) 16 6.8 0 0 2.5 147 35 0.30 3 17 7.1 0.71 0.1 2.5 109 22 0.41 17 18 7.4 7.4 1 2.7 85 19 0.60 26 19 7.3 73 10 2.6 82 24 0.71 111 20 6.8 3400 500 2.7 74 23 1.30 283

(42) As it is clear from the result shown in TABLE 3, when oxygen was supplied into the reaction vessel, the effect of reducing impurities was remarkable.

(43) In Examples, the transferred DC arc plasma device was used. However, the present invention is not limited thereto, and, for example, a radio-frequency induction plasma device or a microwave heating plasma device may be used.

(44) Further, in Examples, oxygen was supplied from the oxygen supply unit different from the dilute gas supply unit, but may be supplied together with a dilute gas.

INDUSTRIAL APPLICABILITY

(45) The present invention is suitably applicable to a manufacturing method of metal powder for manufacturing metal powder by a plasma technique, particularly the method keeping impurity elements from getting mixed in metal powder, thereby obtaining extremely high-purity metal powder.

EXPLANATION OF REFERENCE NUMERALS

(46) 1 Plasma Device

(47) 2 Reaction Vessel

(48) 3 Cooling Tube

(49) 4 Plasma Torch

(50) 5 Anode

(51) 6 Cathode

(52) 7 Plasma

(53) 8 Molten Metal

(54) 9 Crucible Part

(55) 10 Dilute Gas Supply Unit

(56) 11 Oxygen Supply Unit