Provided is a method for producing seed crystals of cobalt powder, sequentially including: a complexing step of adding ammonia, an ammonia compound solution, or both of ammonia and an ammonia compound solution to a cobalt sulfate solution to obtain a solution containing a cobalt ammine sulfate complex; a mixing step of adding a solid to the solution containing the cobalt ammine sulfate complex to form a mixture slurry; a reduction and precipitation step of charging a reaction vessel with the mixture slurry, blowing hydrogen gas into the reaction vessel and reducing cobalt contained in the mixture slurry to obtain a cobalt powder slurry containing cobalt precipitate with a cobalt component precipitated on the surface of the solid as cobalt powder; and a solid-liquid separation step of performing solid-liquid separation on the cobalt powder slurry to obtain cobalt precipitate and a solution after reduction.

Provided is a method for producing seed crystals of cobalt powder, sequentially including: a complexing step of adding ammonia, an ammonia compound solution, or both of ammonia and an ammonia compound solution to a cobalt sulfate solution to obtain a solution containing a cobalt ammine sulfate complex; a mixing step of adding a solid to the solution containing the cobalt ammine sulfate complex to form a mixture slurry; a reduction and precipitation step of charging a reaction vessel with the mixture slurry, blowing hydrogen gas into the reaction vessel and reducing cobalt contained in the mixture slurry to obtain a cobalt powder slurry containing cobalt precipitate with a cobalt component precipitated on the surface of the solid as cobalt powder; and a solid-liquid separation step of performing solid-liquid separation on the cobalt powder slurry to obtain cobalt precipitate and a solution after reduction.

Provided is a method for producing high density nickel powder particularly having a median diameter of 100 to 160 m by controlling a particle size of nickel powder. The method includes: performing an initial operation by charging a pressure vessel equipped with a stirrer with a nickel ammine complex solution containing nickel in the concentration of 5 to 75 g/L together with seed crystals in the amount of 5 to 200 g per liter of the solution, increasing the temperature of the solution, and performing a reduction reaction with hydrogen by blowing hydrogen gas into the pressure vessel, thereby obtaining the nickel contained in the nickel ammine complex solution as nickel powder; and thereafter, performing a specified operation A repeatedly at least once to obtain the nickel powder having the median diameter of 100 to 160 m and a bulk density of 1 to 4.5 g/cm.sup.3.

Provided is a method for producing high density nickel powder particularly having a median diameter of 100 to 160 m by controlling a particle size of nickel powder. The method includes: performing an initial operation by charging a pressure vessel equipped with a stirrer with a nickel ammine complex solution containing nickel in the concentration of 5 to 75 g/L together with seed crystals in the amount of 5 to 200 g per liter of the solution, increasing the temperature of the solution, and performing a reduction reaction with hydrogen by blowing hydrogen gas into the pressure vessel, thereby obtaining the nickel contained in the nickel ammine complex solution as nickel powder; and thereafter, performing a specified operation A repeatedly at least once to obtain the nickel powder having the median diameter of 100 to 160 m and a bulk density of 1 to 4.5 g/cm.sup.3.

Provided is a method for producing coarse particles of high purity nickel powder from a nickel ammine sulfate complex solution using industrially-inexpensive hydrogen gas and fine nickel powder. The method for producing nickel powder, including performing the following processes (1) to (5), wherein a sulfuric acid solution containing nickel and cobalt is subjected to (1) a pH adjusting step, (2) a solvent extraction step, (3) a complexing step for obtaining a nickel ammine sulfate complex solution, (4) a reduction step for obtaining a reduced slurry containing nickel powder, and (5) a solid-liquid separation step of performing solid-liquid separation to obtain nickel powder and a solution after reduction, and solution after reduction repeatedly using the solution after reduction in either or both of (2) the solvent extraction step and (3) the complexing step.

Provided is a method for producing coarse particles of high purity nickel powder from a nickel ammine sulfate complex solution using industrially-inexpensive hydrogen gas and fine nickel powder. The method for producing nickel powder, including performing the following processes (1) to (5), wherein a sulfuric acid solution containing nickel and cobalt is subjected to (1) a pH adjusting step, (2) a solvent extraction step, (3) a complexing step for obtaining a nickel ammine sulfate complex solution, (4) a reduction step for obtaining a reduced slurry containing nickel powder, and (5) a solid-liquid separation step of performing solid-liquid separation to obtain nickel powder and a solution after reduction, and solution after reduction repeatedly using the solution after reduction in either or both of (2) the solvent extraction step and (3) the complexing step.

To provide a metal-based structure or nanoparticles whose homogeneity is not deteriorated and whose sticking formation is easy, and a production method thereof with a high safety. A metal-based structure comprises a hydrogen compound, cluster, or an aggregate thereof, represented by the general formula: M.sub.mH. The M is a metal-based atom. The m is an integer of 3 or more and 300 or less. H is a hydrogen atom.

To provide a metal-based structure or nanoparticles whose homogeneity is not deteriorated and whose sticking formation is easy, and a production method thereof with a high safety. A metal-based structure comprises a hydrogen compound, cluster, or an aggregate thereof, represented by the general formula: M.sub.mH. The M is a metal-based atom. The m is an integer of 3 or more and 300 or less. H is a hydrogen atom.

Provided is a method for producing nickel powder, wherein fine nickel powder serving as seed crystals needed for production of nickel powder is produced from a solution containing a nickel ammine sulfate complex according to the amount needed for the production of the nickel powder. The method for producing nickel powder is characterized in that: the solution containing the nickel ammine sulfate complex, an insoluble solid, and a dispersant are continuously fed into a reaction vessel, followed by stirring to prepare a solution containing a nickel complex ion; hydrogen gas is blown into the prepared solution to reduce the nickel complex ion in the solution containing the nickel complex ion, thereby forming a precipitate of nickel particles on the surface of the insoluble solid; and thereafter the post-reduction solution is extracted from the reaction vessel.

Provided is a method for producing nickel powder, wherein fine nickel powder serving as seed crystals needed for production of nickel powder is produced from a solution containing a nickel ammine sulfate complex according to the amount needed for the production of the nickel powder. The method for producing nickel powder is characterized in that: the solution containing the nickel ammine sulfate complex, an insoluble solid, and a dispersant are continuously fed into a reaction vessel, followed by stirring to prepare a solution containing a nickel complex ion; hydrogen gas is blown into the prepared solution to reduce the nickel complex ion in the solution containing the nickel complex ion, thereby forming a precipitate of nickel particles on the surface of the insoluble solid; and thereafter the post-reduction solution is extracted from the reaction vessel.