Method for producing cobalt metal powder

A method for producing extra fine cobalt metal powder (up to 1.7 FSSS) by the digestion of cobalt pentammine chloride in a dilute ammonium hydroxide solution to form a cobalt-containing precipitate, followed by reducing the precipitate in a hydrogen atmosphere to cobalt metal powder, is improved by: (a) treating the mother liquor with an ion exchange resin to remove cobalt species; (b) stripping the resin with an HCl solution; (c) recovering solid cobalt hexammine chloride from the stripping solution; (d) forming a solution of the solid in water; (e) adding NaOH to the solution to form a cobalt-containing precipitate; and (f) either heating the precipitate in hydrogen to reduce it to cobalt metal powder or recycling the precipitate as a cobalt source for the formation of cobalt pentammine chloride. The final cobalt metal powder is useful, for example, as a starting material in the manufacture of cemented carbides.

TECHNICAL FIELD 
This invention relates to an improved method for obtaining cobalt metal 
powder from a cobalt source, and more particularly relates to an improved 
method for obtaining such powder by the thermal reduction of a precipitate 
obtained by the digestion of a cobalt pentammine chloride solution. 
CROSS REFERENCE TO RELATED APPLICATIONS 
Co-pending U.S. patent application Ser. No. 010,769, filed, Feb. 9, 1979, a 
continuation of Ser. No. 911,595, now abandoned, describes a method for 
obtaining fine particle size cobalt by hydrogen reduction of a precipitate 
obtained from a cobalt pentammine chloride solution. Co-pending U.S. 
patent application Ser. No. 038,973, filed concurrently herewith, 
describes an improvement of the method of Ser. No. 010,769, involving 
removal of cobalt from solution by addition of a metal hydroxide. 
Application 038,972, also filed concurrently herewith, describes a method 
for the production of cobalt metal powder involving the recycling of 
ammonia. Application 038,971, also filed concurrently herewith, describes 
a method for the production of cobalt metal powder from a precipitate 
obtained by treatment of a cobalt ammine complex with a metallic 
hydroxide. Application 038,970, also filed concurrently herewith, 
describes a method for producing cobalt hexammine compounds and cobalt 
metal powder. 
BACKGROUND ART 
According to German Patent No. 2,319,703, it is known to separate cobalt 
from nickel by a method which includes forming pentammine sulfate 
complexes of the two ions in solution. However, it has been found that 
soluble cobalt ammine sulfates can only be reduced while still in 
solution, under pressure, and with the aid of catalysts. Furthermore, the 
resulting cobalt powder is not fine particle size. 
U.S. Pat. No. 4,093,450 to Doyle et al. describes a method for producing 
fine particle size cobalt metal powder by the hydrogen reduction of cobalt 
oxide obtained from a cobalt pentammine carbonate solution. The 
precipitate was formed by heating the solution to drive off ammonia and 
carbon dioxide to form a precipitate of cobalt oxide. This method requires 
a solution of approximately four grams per liter of cobalt to produce a 
sized metal powder having a particle size less than one micron. Note that 
the final resulting particle size is highly dependent on the concentration 
of cobalt employed in the aqueous solution. 
The following patents are directed to the separation of cobalt from other 
cations, especially nickel. The resulting cobalt compounds are not 
disclosed as being sources for forming fine particle size cobalt. 
U.S. Pat. No. 2,879,137 to Bare et al. discloses the treatment of an 
ammoniacal ammonium carbonate solution, obtained from leaching an ore and 
containing nickel wherein the cobalt present in the cobaltic state is 
treated with an alkali metal or alkaline earth metal hydroxide under 
controlled temperature conditions to precipitate the nickel free of 
cobalt. 
U.S. Pat. No. 3,928,530 to Bakker et al. discloses a method for the 
separation of nickel and cobalt by forming pentammine chloride complexes 
and solution containing a high concentration of ammonium chloride, and 
precipitating cobalt pentammine chloride. 
In German Patent No. 1,583,864, cobalt is recovered from scrap by digestion 
of the scrap in HCl and MgCl.sub.2 solution, followed by removal of iron 
and chromium impurities by precipitation at a moderately acid pH followed 
by extracting a cobalt chloride complex with a long chain tertiary ammine 
in an aromatic solvent. 
U.S. Pat. No. 4,108,640 to Wallace discloses a method for recovering 
metallic cobalt from an aqueous ammoniacal solution wherein the solution 
is contacted with a water immiscible liquid ion exchange reagent dissolved 
in an inert organic diluent to selectively extract the other metal from 
the solution and produce an organic extract loaded with the other metals 
in an aqueous cobalt bearing raffinate substantially free of the other 
metals. 
DISCLOSURE OF THE INVENTION 
In a method for the production of cobalt metal powder by the thermal 
reduction of a cobalt-containing precipitate obtained by the digestion of 
a cobalt pentammine chloride solution, the efficiency of the method is 
improved by: (a) contacting the mother liquor from at least the digestion 
step with an ion exchange resin to remove residual cobalt species; (b) 
stripping the resin with a solution of a strong mineral acid such as HCl; 
(c) separating solid cobalt hexammine chloride from the stripping 
solution; (d) forming an aqueous solution of the solid cobalt hexammine 
chloride; (e) adding a metallic hydroxide to the solution to form a 
cobalt-containing precipitate; and (f) either heating the precipitate in a 
reducing atmosphere to convert it to cobalt metal powder, or recycling the 
precipitate. 
The final cobalt metal powder is useful, for example, as a starting 
material in the manufacture of cemented carbides.

BEST MODE FOR CARRYING OUT THE INVENTION 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above described drawing. 
The method for producing fine particle size cobalt metal powder, herein 
referred to as the "basic" method, of which the present invention is an 
improvement, is described and claimed in copending patent application Ser. 
No. 010,769, filed Feb. 9, 1979, and assigned to the present assignee. 
However, a brief summary of one embodiment of that method will be set 
forth to aid the practitioner. 
Cobalt pentammine chloride in solution is obtained by a method including 
the steps of: (1) digestion of a cobalt source in hydrochloric acid 
solution to obtain a solution of typically about 60 to 150 grams per liter 
of cobalt; (2) addition of ammonium hydroxide to result in a concentration 
of about 100 to 150 grams per liter of ammonium chloride and a pH of about 
9.2 to 10; (3) oxidation of the cobaltous ion to cobaltic to form soluble 
cobaltic pentammine chloride, such as by aeration or contact with an 
oxidizing agent such as hydrogen peroxide or a combination of these, 
typically for a time of about 1 to 10 hours, and preferably about 2 to 8 
hours; and (4) optional heating of the solution to a moderate temperature, 
for example about 80.degree. to 90.degree. C., in order to dissolve 
substantially all of the cobalt pentammine chloride. 
Cobalt metal powder is then obtained by: (1) digesting the cobalt 
pentammine chloride solution, typically for about 2 to 10 hours at a 
temperature of about 80.degree. C. to 105.degree. C., in order to 
decompose the cobalt pentammine chloride and form a cobalt-containing 
precipitate; (2) separating the precipitate from the solution; and (3) 
heating the precipitate in a reducing atmosphere for a time and 
temperature sufficient to reduce the precipitate to cobalt metal powder, 
typically having a Fisher (FSSS) of 1.7 or less. 
Depending upon the composition of the cobalt source and the purity desired 
for the final cobalt metal powder, several additional steps may be carried 
out. For example, after the initial digestion of the cobalt source and 
prior to the addition of ammonium hydroxide to the solution, it may be 
necessary to remove insoluble sludge, typically containing tantalum, 
titanium and tungsten from cobalt sources including significant amounts of 
scrap or sludge from cemented carbide recovery operations. Separation of 
precipitates or sludges from solutions in any of the above or subsequent 
steps may be accomplished by filtering followed by washing the filtrate. 
Cation impurities may be removed from the cobalt pentammine chloride 
solution prior to digestion to obtain the cobalt-containing precipitate by 
the addition of sufficient hydrochloric acid to precipitate solid cobalt 
pentammine chloride, followed by separation of the solid and addition to 
an (e.g. 1 to 6%) ammonia solution. After dissolving cobalt pentammine 
chloride in ammonia solution and prior to digestion, it may again be 
necessary to remove insoluble sludge. At this stage, cation impurities 
other than iron in the solution will usually be in the range of about 100 
parts per million to 1 weight percent. Iron will usually be less than 1000 
parts per million. Cationic impurities of less than 100 parts per million 
may also be achieved by one or more optional "recrystallizations" (i.e., 
acid precipitation followed by ammonia dissolution) of the cobalt 
pentammine chloride prior to digestion to form the cobalt-containing 
precipitate. 
Reduction is typically carried out in a hydrogen atmosphere for a time of 
from about 1 to 6 hours at a temperature within the range of about 
350.degree. C. to 600.degree. C. 
In the above-described basic method, mother liquor from the acid 
precipitation steps and the second digestion step contain various cobalt 
species in solution. Such species may include cobalt pentammine chloride, 
due to incomplete conversion to the cobalt-containing precipitate, and 
cobalt haxammine chloride, which is incidentally formed during formation 
of the pentammine species, and is not converted to precipitate during 
digestion. The mother liquor typically contains up to 0.5 weight percent 
of cobalt. 
Accordingly, the present invention is an improvement of the above-described 
basic method wherein the mother liquor from the second digestion step and 
any acid precipitation steps is treated to convert solid cobalt hexammine 
chloride to a cobalt-containing precipitate, and the precipitate either 
reduced to cobalt metal powder or recycled by adding it to the cobalt 
source for repetition of the basic method. 
Referring now to the Drawing, which is a flow diagram of one embodiment of 
the overall method, the basic method is depicted as a multi-step method on 
the left-hand side of the diagram, beginning with digestion of the cobalt 
source in HCl solution and ending with heating to reduce the 
cobalt-containing precipitate to cobalt metal powder. The acid 
precipitation, "recrystallization" and the second digestion step of this 
method leaves residual cobalt species in the mother liquor. 
According to one embodiment of an improvement of this method, the mother 
liquors from the above steps are combined, the pH adjusted to about 5.5 to 
7, and any solids are removed by filtering. The mother liquor is then 
contacted with a weak cationic ion exchange resin such as Duolite C-464 or 
Duolite C-433, both weak carboxylic cationic resins, in order to remove 
cobalt species. The resin is periodically stripped with a strong mineral 
acid solution, such as an HCl solution containing about 2 to 10 weight 
percent HCl. As the concentration of HCl in the stripping solution 
increases to about 3 to 10 percent, cobaltic hexammine chloride becomes 
insoluble and precipitates as a yellow solid. This solid cobaltic 
hexammine chloride is then separated from the stripping solution, 
dissolved in water, typically in concentration of about 1 to 5 weight 
percent, and the resultant solution treated with an alkali or alkaline 
earth hydroxide such as NaOH, typically in the amount of about 3 to 15 
weight percent, to form the cobalt-containing precipitate. This 
precipitate, depending upon its impurity content and the final end use 
envisioned for the cobalt metal powder, may after separation, either be: 
heated in a reducing atmosphere to form cobalt metal powder; redissolved 
in HCl and reprecipitated with hydroxide to remove impurities, followed by 
reducing to cobalt metal powder; or recycled by adding it to the cobalt 
source for reprocessing. 
The mother liquor from the stripping solution which remains after removal 
of the solid cobalt hexammine chloride may be combined with the mother 
liquors from subsequent acid precipitation and digestion steps for 
treatment with the ion exchange resin. 
The cobalt species-depleted effluent from the ion exchange column is 
typically treated to remove NH.sub.3 prior to being discarded. 
EXAMPLE 
Mother liquors from the acid precipitation steps and the second digestion 
step of the basic process were combined, the pH adjusted to about 5.8 to 
6.3 with NH.sub.4 OH solution, and the resulting solution filtered to 
remove insolubles. The filtered solution was then loaded onto an ion 
exchange column containing Duolite C-433 weak carboxylic cationic exchange 
resin, manufactured by Diamond Shamrock, Redwood, Calif. The resin had 
previously been converted to the ammonia cycle by treatment with a 2 to 6 
weight percent NH.sub.4 OH solution. The effluent from the column, which 
contained less than than 10 parts per million of cobalt, was then loaded 
onto another column to reduce the cobalt level even further. The effluent 
from the second column contained less than 5 parts per million of cobalt. 
The cobalt was then removed from the resin with a stripping solution of 
about 4 to 10 weight percent solution of HCl. This stripping solution was 
used repeatedly to strip cobalt, by periodically replenishing the HCl 
consumed during each stripping cycle. After ten to fifteen cycles, solid 
material precipitated from the stripping solution. A 21.9 gram sample of 
this solid was separated from the stripping solution by filtration. To the 
solid was added 1.3 liters of water, the resultant slurry was heated to 
about 80.degree. C., and the pH adjusted to about 7 with concentrated 
NH.sub.4 OH. Insoluble impurities were filtered, and the resultant cobalt 
hexammine chloride solution was treated with 0.9 liters of concentrated 
HCl solution at 80.degree. C., cooled to 25.degree. C., and the mother 
liquor from the insoluble solid cobalt hexammine chloride removed. The 
solid was washed with 6 Normal HCl solution to yield 20.7 grams of cobalt 
hexammine chloride containing the following cationic impurities: 
______________________________________ 
ppm 
______________________________________ 
Ca 4.0 
Cu 3.0 
Mg 2.1 
Mn 3.0 
Ni 10 
Si 14 
Cr 8.0 
Fe 10 
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This solid was dissolved in about 1.2 liters of hot (90.degree. C.) water, 
and the resulting hot solution was treated with 0.11 liters of 50 weight 
percent sodium hydroxide solution over a period of 9 hours to give 20.2 
grams of insoluble cobalt-containing precipitate, for a yield of 92 
percent. This precipitate was heated at 400.degree. C. in hydrogen 
atmosphere to obtain extra fine particle size cobalt metal powder having a 
Fisher Sub Sieve Size (FSSS) of about 1.0. 
While there has been shown and described what are at present considered the 
preferred embodiments of the invention it will be obvious to those skilled 
in the art that various changes and modifications may be made therein 
without departing from the invention as defined by the appended claims. 
INDUSTRIAL APPLICABILITY 
The method described and claimed herein is particularly useful in the 
formation of extra fine particle size, (FSSS) of up to 1.7) high purity 
(cation impurities of less than 100 parts per million) cobalt metal 
powder, which is useful, for example, as a starting material in the 
formation of cemented carbides, e.g., tungsten carbide.