Purification of molybdenum

A method is disclosed for purifying molybdenum which involves adding to an ammoniacal ammonium molybdate solution containing the impurities of phosphorus and arsenic with the phosphorus concentration being from about 0.01 to about 0.12 g/l, a soluble magnesium salt to form a precipitate comprising magnesium ammonium salts of the phosphorus and arsenic, and to form a purified ammonium molybdate solution. The amount of the magnesium salt is sufficient to result in a concentration of from about 0.005 to about 0.04 moles/l in the ammoniacal ammonium molybdate solution. The resulting purified ammonium molybdate contains no greater than about 0.01 g P/l. The precipitate is separated from the purified solution which is then contacted with a chelating cation exchange resin supplying a sufficient amount of a cation to result in removal of the major portion of the magnesium ions from the purified solution and form a further purified ammonium molybdate solution.

BACKGROUND OF THE INVENTION 
This invention relates to a method for purifying molybdenum which involves 
precipitating magnesium-ammonium salts of arsenic and phosphorus from an 
ammoniacal ammonium molybdate solution followed by removal of the 
magnesium from the solution. 
Sources of molybdenum such as impure or technical grade molybdenum trioxide 
are often contaminated with phosphorus and/or arsenic. These impurities 
are difficult to remove and are often present as contaminants in 
molybdenum compounds where they result in inferior quality of the 
molybdenum products. For example, phosphorus in ammonium dimolybdate shows 
up in the subsequently produced molybdenum metal. Phosphorus at a level of 
about 40 ppm in molybdenum powder causes a decrease in the rolling 
efficiency of molybdenum. 
Therefore, a method to remove impurities such as phosphorus and arsenic 
from molybdenum would be advantageous because it would allow processing of 
a wide variety of molybdenum sources. 
U.S. Pat. No. 3,829,550 discloses a process for producing a high purity 
molybdenum trioxide and/or ammonium molybdate product whereby an oxidized 
molybdenite concentrate is subjected to an ammonium hydroxide leaching 
step including a digestion phase in which an oxidation of some of the 
impurities therein, particularly iron, is effected, resulting in a 
coprecipitation of iron and aluminum hydroxide, together with other 
impurities including lead, bismuth, tin, arsenic, phosphorus, soluble 
silica, and the like. The resultant aqueous solution containing ammonium 
molybdate is filtered and thereafter crystallized, followed by calcining 
to produce a high purity molybdenum trioxide. This molybdenum trioxide can 
be further purified by digestion in a dilute nitric acid solution to 
effect a further leaching of residual contaminants, and thereafter the 
molybdenum trioxide is redissolved in an aqueous ammonium hydroxide 
solution which is filtered and subsequently crystallized to produce a high 
purity ammonium molybdate. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of this invention, there is provided a method 
for purifying molybdenum which involves adding to an ammoniacal ammonium 
molybdate solution containing the impurities of phosphorus and arsenic 
with the phosphorus concentration being from about 0.01 to about 0.12 g/l, 
a soluble magnesium salt to form a precipitate comprising magnesium 
ammonium salts of the phosphorus and arsenic, and to form a purified 
ammonium molybdate solution. The amount of the magnesium salt is 
sufficient to result in a concentration of from about 0.005 to about 0.04 
moles /l in the ammoniacal ammonium molybdate solution. The resulting 
purified ammonium molybdate contains no greater than about 0.01 g P/l. The 
precipitate is separated from the purified solution which is then 
contacted with a chelating cation exchange resin supplying a sufficient 
amount of a cation to result in removal of the major portion of the 
magnesium ions from the purified solution and form a further purified 
ammonium molybdate solution.

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 described drawing and description of some of the aspects of the 
invention. 
This invention relates to a method for removing phosphorus and arsenic from 
an ammoniacal ammonium molybdate solution by adding to the solution a 
soluble magnesium salt to result in the resulting solution having a 
magnesium content of from about 0.005 to about 0.04 moles/l to precipitate 
the magnesium ammonium salts of phosphorus and arsenic and thereafter 
removing the magnesium ions from the purified ammonium molybdate solution 
by contacting the solution with a chelating cation exchange resin. The 
removal of magnesium ions from the purified ammonium molybdate solution 
allows the process to be commercially feasible. 
U.S. Pat. No. 4,273,745 discloses a process for recovering molybdenum 
trioxide from ammonium molybdate solutions containing U.sub.3 O.sub.8 by 
first precipitating phosphorus and then acidifying and precipitating 
ammonium polymolybdate which is converted to molybdenum trioxide. This 
patent does not teach the criticality of the concentration of the water 
soluble phosphours precipitataing compound in ammoniacal ammonium 
molybdate, in particular of a magnesium water soluble salt, nor does it 
teach or suggest a critical pH range to precipitate magnesium ammonium 
phosphate, whereas these are essential features of the present invention. 
Also this patent does not provide for the removal of magnesium ions from 
the solution after precipitation of the phosphorus which is another 
feature of this invention. 
U.S. Pat. No. 4,278,644 discloses recovery of refractory and base metals 
from superalloy scrap. It provides detailed methods for recovery of Mo, W, 
Cr, and V. While phosphate removal is discussed, it does not claim a 
phosphorus removal step. The molybdenum containing solutions of U.S. Pat. 
No. 4,278,644 are basically different from those of the present invention. 
For example, in U.S. Pat. No. 4,278,644 the solution is an alkali metal 
solution containing molybdenum, tungsten, vanadium, chromium, carbonate, 
and about 2.5 g P/l typically. In the present invention, the solution is 
an ammoniacal ammonium molybdate solution and any W, V, or Cr is present 
in trace quantities, and the P concentration of typically from about 0.01 
to about 0.12 g/l. In U.S. Pat. No. 4,278,644, the acceptable level of P 
in the phosphorus purified solution is about 0.02 g/l whereas in the 
present invention, P in the purified solution is no greater than about 
0.01 g/l. U.S. Pat. No. 4,278,644 does not provide for a magnesium removal 
step as in the present application. 
U.S. Pat. No. 4,320,094 also discloses a process for recovery of refractory 
and base metals from superalloy scrap. This process is different from the 
present invention for essentially the same reasons discussed above for 
U.S. Pat. No. 4,278,644. 
The molybdenum to be purified is typically an impure molybdenum trioxide or 
a technical grade molybdenum trioxide. 
The molybdenum trioxide is dissolved in sufficient ammonium hydroxide so 
that the pH of the resulting ammoniacal ammonium molybdate solution is 
from about 8.5 to about 11 and preferably from about 9 to about 10. The pH 
ranges are necessary for the susequent precipitation of the 
magnesium-ammonium salts of arsenic and phosphorus. The solution contains 
typically from about 180 to about 240 g Mo/l, and from about 0.01 to about 
0.12 g P/l. The arsenic content can vary, but is typically greater than 
about 0.01g/l and most typically in the range of from about 0.01 g/l to 
about 0.06 g/l. The solution can also contain 0.02 g W/l, trace amounts of 
Ca and Cu, and other impurities. A soluble magnesium salt is added to the 
ammoniacal ammonium molybdate solution to form a precipitate comprising 
magnesium-ammonium salts containing a portion of the phosphorus and/or 
arsenic and a purified ammonium molybdate solution containing magnesium 
ions. The typical magnesium salts are magnesium chloride, magneisum 
nitrate, and magnesium sulfate. Magnesium nitrate is especially preferred. 
It is critical that the amount of magnesium be sufficient to result in a 
concentration in the ammoniacal ammonium molybdate solution of from about 
0.005 to about 0.04 moles Mg/l, and preferably from about 0.008 to about 
0.04 moles Mg/l. When the Mg content of the ammoniacal ammonium molybdate 
solution is least 0.005 moles/l, the P content of the resulting purified 
solution is no greater than about 0.01 g/l. When the magnesium content is 
at least about 0.008 moles/l, the P content of the resulting purified 
solution is no greater than about 0.005 g/l, and the As content is no 
greater than about 0.01 g/l. 
The precipitate is then separated from the resulting partially purified 
ammoniacal ammonium molybdate solution by any standard technique such as 
filtration. Along with this precipitate can be residual solids which may 
be present from the original dissolution of the molybdenum trioxide in the 
ammonium hydroxide. 
This purified ammonium molybdate solution contains magnesium ions from the 
magnesium salt. These ions are removed to further purify the molybdenum as 
follows. 
The solution is contacted with a chelating cation exchange resin supplying 
a sufficient amount of a cation to remove the major portion of the 
magnesium ions and form a further purified ammonium molybdate solution. 
The chelating cation exchange resin is an iminodiacetate resin. The 
preferred chelating cation exchange resin is supplied by Mobay under the 
trade name of Lewatit TP-207. Other resins such as Rohm and Haas IRC-718 
and Biorad Chelex 100 can also be used. The cation is typically the 
ammonium ion which is exchanged for the magnesium ions of the solution. 
The actual cation exchange operation is done by standard techniques 
preferably by passing the solution containing the magnesium ions through a 
column which is packed with the resin having as the cation the ammonium 
ion. In actual practice, by analyzing the effluent from the column for 
magnesium, it is determined when the column is loaded. 
The resulting spent resin which now contains magnesium cations can be 
regenerated by washing with acid to remove any metal ions, followed by 
washing with ammonium hydroxide to restore the ammonium ion as the cation. 
The resulting further purified ammonium molybdate solution can now be 
processed by standard methods such as by evaporative crystallization to 
obtain ammonium dimolybdate which can then be reduced to molybdenum metal. 
The above described method is a way to purify molybdenum of the hard to 
remove common contaminants of phosphorus and arsenic. 
In the molybdenum metal, phosphorus levels above about 40 weight parts per 
million based on Mo content, reduce the rolling efficiency of the 
molybdenum. By the method of the present invention, treating a solution 
containing from about 180 to about 240 g Mo/l with a magnesium salt at a 
concentration of about 0.01 moles Mg/l, typically results in a P content 
in solution of no greater than about 0.006 g/l. The Mo metal produced from 
this type of solution typically contains about 25 weight ppm P on a Mo 
basis. This is a sufficiently low P content so that problems in processing 
the Mo metal due to P contamination do not occur. 
To more fully illustrate this invention, the following nonlimiting examples 
are presented. 
EXAMPLE 1 
To about 100 ml of an ammoniacal ammonium molybdate solution containing 
about 180 g Mo/l and about 0.076 g P/l is added about 1 ml of a 1.0 molar 
MgCl.sub.2.6H.sub.2 O solution which supplies about 0.01 moles of Mg/l. A 
precipitate forms and is removed by filtration. The resulting purified 
solution contains &lt; about 0.005 g P/l. The molybdenum concentration 
remains at about 180 g/l. When the molybdenum is processed to molybdenum 
metal, the P content is about 28 weight ppm based on Mo. 
EXAMPLE 2 
About 1150 gallons of ammonium molybdate solution at a pH of about 8.8 
contains about 240 g Mo/l, about 0.077 g P/l, and about 0.050 g As/l. When 
about 3 gallons of about 1.1 M MgCl.sub.2.6H.sub.2 O is added to result in 
a concentration of about 0.003 moles of Mg/1 in the ammonium molybdate 
solution at about 28.degree. C., a precipitate does not form. Ammonium 
hydroxide is added to increase the pH to about 9.8 and about 3 more 
gallons of 1.1 M MgCl.sub.2.6H.sub.2 O are added to result in a magnesium 
concentration of about 0.006 moles per liter. A precipitate forms and is 
removed by filtration. The phosphorus content of the resulting purified 
solution is about 0.01 g/l. 
EXAMPLE 3 
An ammoniacal ammonium molybdate solution contains about 200 g Mo/l, about 
0.064 g P/l, and about 0.063 g As/l. About 6 gallons of about 1.1 M 
MgCl.sub.2.6H.sub.2 O is added to about 2000 gallons of the molybdate 
solution at about 24.degree. C. to result in the solution having a 
magnesium concentration of about 0.003 moles per liter. A precipitate 
forms and is removed by filtration. The phosphorus content of the 
resulting partially purified ammonium molybdate solution is about 0.020 
g/l, and the arsenic content is about 0.045 g/l. The precipitation removes 
about 69% of the phosphorus and about 29% of the arsenic. The resulting 
partially purified solution is passed through Lewatit-TP-207 chelating 
cation exchange resin in the ammonium form. The resulting purified 
molybdate solution is essentially free of magnesium after the ion 
exchange. 
EXAMPLE 4 
The molybdate solution of Example 3 is used in this example. Magnesium is 
added as 1.0 M Mg(NO.sub.3).sub.2.6H.sub.2 O in varying amounts: 2 ml, 4 
ml, and 6 ml of the magnesium solution are added to 3-200 ml portions of 
molybdate solution. The magnesium additions provide 0.01, 0.02, and 0.03 
moles Mg/l respectively. Precipitates formed in each case which are 
filtered from the respective solutions. The resulting solutions are 
analyzed for phosphorus and arsenic. Each Mg additive equal to or above 
about 0.01 moles/l lowers the P content to less than about 0.005 g/l and 
the As content to less than about 0.01 g/l. 
The results of Examples 3 and 4 are included in FIG. 1 and in Table 1 that 
follows. FIG. 1 shows how the percent P and As removed increase with 
increasing Mg concentration. 
TABLE 1 
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Precipitation of Phosphorus and Arsenic 
Given: Ammoniacal ammonium molybdate solution 
containing about 200 g Mo/l, about 0.064 g P/l and 
about 0.063 g As/l. 
Example 3 Example 4 
Moles Mg/l Added 
0.003 0.01 0.02 0.03 
______________________________________ 
impurity level 
after precipitation: 
g/ P/l 0.020 &lt;0.005 &lt;0.005 &lt;0.005 
g As/l 0.045 0.007 0.003 &lt;0.003 
% P removed 69 100 100 100 
% As removed 29 88 95 &gt;95 
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EXAMPLE 5 
An ammonacal ammonium molybdate solution contains about 200 g Mo/l, about 
0.105 g P/l, and about 0.04 g As/l. The pH is about 9.3 at about 
22.degree. C. Mg ion is added as an aqueous solution of 
MgCl.sub.2.6H.sub.2 O. Additions are made so that concentrations range 
from about 0.001 moles Mg/l to about 0.04 moles Mg/l. The solutions are 
stirred for about 1 hour and then allowed to stand for about 3 hours. The 
resulting precipitates are filtered from each solution. The solutions are 
then analyzed for As concentration. To determine effectiveness of removal 
of P, the solutions are evaporated to dryness to form ammonium 
dimolybdate, ADM, which is then reduced to molybdenum metal. The 
phosphorus content of the resulting Mo metal is measured and is reported 
in Table 2. 
TABLE 2 
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P and As Precipitation as a Function of Mg Concentration 
Moles 
Sample Mg/l In Purified Solution 
In Mo Powder 
# Added g As/l weight ppm P 
______________________________________ 
Control 
None 0.04 530 
1 0.001 0.04 320 
2 0.003 0.03 120 
3 0.005 0.02 45 
4 0.008 0.01 25 
5 0.01 0.01 22 
6 0.02 0.01 22 
7 0.03 0.01 23 
8 0.04 0.001 20 
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It can be seen that increasing the Mg content to abaout 0.008 moles/l, 
results in about 25 weight ppm in the Mo metal. This is an acceptable P 
level. Increasing the amount of magnesium results in an increased removal 
of both P and As. 
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.