Electrolytic method for producing ammonium paratungstate from cemented tungsten carbide

A method is disclosed for producing ammonium paratungstate from cemented tungsten carbide which comprises subjecting the cemented tungsten carbide to electrolysis by passing about 0.5 to about 20 volts through an ammoniacal solution selected from the group consisting of ammonium hydroxide-ammonium chloride solution, ammonium hydroxide-ammonium carbonate solution, and ammonium hydroxide-ammonium sulfate solution, wherein platinum and the cemented tungsten carbide serve as the anode and wherein the anode is immersed in the solution, to decompose the cemented tungsten carbide and form a solution of ammonium tungstate from which is crystallized ammonium paratungstate.

BACKGROUND OF THE INVENTION 
This invention relates to a method for producing ammonium paratungstate 
(APT) from cemented tungsten carbide by electrodissolution. Use of this 
method avoids the use of processes which involve dissolution of the 
tungsten in alkali or the use of expensive reagents. 
Ammonium paratungstate is normally prepared by any one of several well 
known techniques. One technique involves the dissolution of tungstic acid 
in ammonium hydroxide to produce ammonium tungstate followed by filtration 
and evaporation to crystallize ammonium paratungstate. Another technique 
involves addition of ammonium chloride to sodium tungstate solution to 
precipitate ammonium paratungstate and leave sodium chloride in solution. 
Still another technique involves the extraction of a metatungstate species 
with an organic extracting agent followed by stripping with ammonium 
hydroxide to produce ammonium tungstate which is evaporated to produce 
ammonium paratungstate crystals. Another technique involves addition of 
ammonium hydroxide to ammonium metatungstate solution to precipitate 
ammonium paratungstate crystals. 
The disadvantages of these processes are that they all require preparation 
of ammonium tungstate by dissolution or oxidation of the tungsten starting 
material. Tungsten ores, tungsten scrap, or tungsten carbide scrap must be 
digested in strong alkali directly or first oxidized to tungstic oxide and 
then digested in strong alkali. The alkali is usually sodium hydroxide. 
This produces sodium tungstate. To produce ammonium paratungsten the 
sodium must be eliminated by precipitation of tungstic acid by the 
addition of acid, which is usually hydrochloric acid. The resulting 
tungstic acid is then washed until it is free of sodium, which is 
difficult to do because tungstic acid becomes colloidal during washing, 
making it very difficult to wash and to handle. The addition of ammonium 
chloride to sodium tungstate requires the preparation of the sodium 
tungstate and the yield of ammonium paratungstate is usually low, and the 
ammonium paratungstate contains sodium. 
The extraction of a tungsten species, using an organic extracting agent 
requires a large investment in equipment and again requires preparation of 
sodium tungstate for extraction. The preparation of ammonium paratungstate 
from metatungstate is very expensive since metatungstate is usually 
prepared from ammonium paratungstate. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the invention, there is provided a method 
for producing ammonium paratungstate from cemented tungsten carbide which 
comprises subjecting the cemented tungsten carbide to electrolysis by 
passing about 0.5 to about 20 volts through an ammoniacal solution 
selected from the group consisting of ammonium hydroxide-ammonium chloride 
solution, ammonium hydroxide-ammonium carbonate solution, and ammonium 
hydroxide-ammonium sulfate solution, wherein platinum and the cemented 
tungsten carbide serve as the anode and wherein the anode is immersed in 
the solution, to decompose the cemented tungsten carbide and form a 
solution of ammonium tungstate from which ammonium paratungstate is 
crystallized. 
DETAILED DESCRIPTION OF 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 description of some of the aspects of the invention. 
The present invention affords a method for preparation of ammonium 
paratungstate directly from cemented tungsten carbide which is usually 
scrap cemented tungsten carbide. The advantages of this method over prior 
methods are there is no oxidation or firing of starting material to 
tungstic oxide and no digestion in sodium hydroxide is required. Also 
tungstic acid is not needed, nor is any extraction process. 
The starting material is cemented tungsten carbide, that is tungsten 
carbide with cobalt, iron or nickel as binder. Typically the cemented 
tungsten carbide contains cobalt at a level of about 3% to about 12% by 
weight and most typically about 6% by weight. Usually, the material is 
scrap cemented tungsten carbide such as old cutting tools and carbide 
parts. 
The method involves electrodissolution of the tungsten in an ammoniacal 
solution which serves as the electrolyte. The electrodissolution technique 
is well known in the art. The solution can be ammonium hydroxide and 
ammonium chloride or ammonium hydroxide and ammonium carbonate, or 
ammonium hydroxide and ammonium sulfate. Most typically the solution is 
made up of reagent grade ammonium hydroxide, typically about 28% by weight 
ammonium hydroxide. The chloride, sulfate, or carbonate concentration is 
sufficient to make the solution conductive but no so concentrated that it 
interferes with the subsequent crystallization of APT. Typically the 
solution contains about 5 to about 25% by weight and most typically about 
10% by weight of the ammonium chloride, ammonium sulfate or ammonium 
carbonate in the ammonium hydroxide. 
The cemented tungsten carbide material is attached to a piece of platinum 
wire and these serve as the anode. The anode is immersed in the solution. 
The cathode is usually a piece of platinum foil which is immersed in the 
solution. 
About 0.5 to 20 volts are passed through the solution. The amperage depends 
on the size of the carbide pieces. The larger the piece, the larger the 
amperage. Most typically the amperage is about 0.5 to about 30 amps. This 
results in the tungsten carbide decomposing and an ammonium tungsten 
solution being formed. The binder from the cemented tungsten carbide is 
electroplated and recovered on the cathode. 
The elements of Nb, Ta and Ti, if present in the starting carbide material 
are insolubles in the solution. These are removed usually by filtration. 
Ammonium paratungstate is then crystallized from the solution. This is done 
by evaporating the solution usually to about one-third of its original 
volume and taking a crystal crop. Generally the purity of this crystal 
crop is at least about 99.0%. 
Other crystal crops can then be taken or the balance of the solution can be 
evaporated to dryness. When the balance is evaporated to dryness the 
product is fired to remove impurities from the solution such as ammonium 
chloride, if necessary.

To more fully illustrate this invention, the following nonlimiting example 
is presented. All parts, portions and percentages are by weight unless 
otherwise stated. 
EXAMPLE 
About 15 parts of ammonium chloride are dissolved in about 140 to about 150 
parts of ammonium hydroxide. About 22.1433 parts of cemented tungsten 
carbide scrap material is attached to platinum wire to serve as the anode. 
A piece of platinum foil serves as the cathode. Both anode and cathode are 
immersed in the ammonium chloride-ammonium hydroxide solution. A current 
is rung through the solution as follows: 
______________________________________ 
Time Volts Amps Remarks 
______________________________________ 
0 13.0 3.5 
12 min. 
12.0 5.0 Solution turning brownish- 
yellow 
15 min. 
11.5 7.5 More ammonium hydroxide is 
added 
40 min. 
11.5 9.0 80.degree. C. 
130 min. 
11.5 10.0 
180 min. 
11.5 8.0 
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At the end of three hours the tungsten carbide has lost about 14.9663 
parts. The cathode has cobalt metal plated on it. A grayish precipitate is 
present in the solution which is composed of the oxides of tantalum, 
niobium and titanium. This is filtered out leaving a pinkish solution of 
ammonium tungstate. This solution is evaporated to about one-third of its 
original volume to crystallize or precipitate ammonium paratungstate. A 
spectrographic analysis of this ammonium paratungstate shows a fairly pure 
product. The remainder of the solution or mother liquor is evaporated to 
dryness and fired at about 400.degree. C. to remove ammonium chloride and 
recover the tungsten as tungstic oxide. Results of the spectrographic 
analysis are given in the Table below. 
TABLE 
______________________________________ 
Product evaporated 
First crystal crop 
to dryness 
______________________________________ 
Al 5-50 ppm 0.01-0.1% 
B 10-100 ppm 
Ca 1-10 ppm 10-100 ppm 
Co 0.1-1.0% 1-10% 
Cr 10-100 ppm 
Cu 5-50 ppm 
Fe 10-100 ppm 10-100 ppm 
Mg 1-10 ppm 0.01%-0.1% 
Mn 10-100 ppm 
Mo 10-100 ppm 10-100 ppm 
Ni 1-10 ppm 0.05%-0.5% 
Si 1-10 ppm 0.05%-0.5% 
Ti 5-50 ppm 
W 10-100% 10-100% 
______________________________________ 
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 scope of the invention as defined by 
the appended claims.