Removal of nickel from cobalt and manganese

This invention relates to the removal of nickel, iron and copper co-dissolved with cobalt or manganese, or cobalt and manganese in water so that cobalt, manganese, or cobalt and manganese can be precipitated as carbonates and then converted to their acetates sufficiently low in nickel, iron and copper so that said acetates can be recycled to the liquid phase air oxidation of a di- or trimethyl benzene to its corresponding di- or tricarboxylic acid without material inhibition of such oxidation.

BACKGROUND OF THE INVENTION 
British Pat. No. 1,413,488 published Nov. 12, 1978 and U.S. Pat. No. 
3,673,154 indicate that the presence of iron, chromium, nickel and copper 
are undesirable contaminants of cobalt, or manganese, or cobalt and 
manganese oxidation catalyst metals used for the liquid phase oxidation of 
m- or p-xylene with air to the respective iso- or terephthalic acid. 
In the production of those acids from the oxidation of the respective 
xylenes, the British Patent teaches a method of metal catalyst recovery 
which, for example, recovers cobalt containing no more than 120 ppm of 
copper, 1600 ppm of iron, 10,000 ppm of nickel and 10,000 ppm chromium on 
a weight basis and is said to be suitable for recycle to the oxidation 
without material inhibition of the foregoing xylene. The method of the 
British Patent comprises extracting the residue left after iso- or 
terephthalic acid product separation and removal of reaction solvent (e.g. 
acetic acid) with aqueous alkali (e.g., sodium) carbonate at a pH of 7-8 
in the presence of air or up to 9.5 in the absence of oxygen which would 
at the higher (8 to 9.5) pH cause the formation of manganese oxide. After 
removal of aqueous extract solution there remains as a solid residue a 
mixture of carbonates of iron, manganese and cobalt. Said mixed metal 
carbonate residue is dissolved in 1.05 to 1.2 times the stoichiometric 
amount of acetic acid, preferably acetic acid containing 40 to 50 weight 
percent water, required to convert the metal carbonates to their acetates. 
Thereafter the solution is heated to distill off acetic acid until a 
solution pH of from 4.5 to 5.8 is reached at which pH an insoluble form of 
iron precipitates. Such method is illustrated as recovering 93.8% of 
cobalt and 97.9% of manganese (in their dissolved acetates in acetic acid) 
with an iron content of 0.575 ppm of the cobalt recovered from a starting 
residue (before aqueous sodium carbonate extraction) having an iron 
content of 14300 ppm of the cobalt content. 
The above United States Patents heats to 93.degree. C. a residue containing 
acetic acid, 0.58 to 1.7 weight percent cobalt, 84 to 270 ppm of iron, 16 
to 51 ppm of chromium, 13 to 40 ppm of nickel and acetic acid to evaporate 
acetic acid until the concentrated solution has a pH above 3. Such 
concentration causes a form of iron and a form of chromium to precipitate 
to the extent that 97% of the iron and 65% of the chromium are removed. 
However, this method did not remove nickel. 
Recently in our laboratories, it has been found that in the neat oxidation 
of liquid o-xylene to o-phthalic acid can be inhibited by copper, or iron, 
or nickel in rather low amounts based on the cobalt metal oxidation 
catalyst. The oxidation retarding effects of said three metals in 
concentrations per million weight parts (ppm) of cobalt are shown in TABLE 
I to follow. The retardation effect is shown as mole percent of 
theoretical oxygen consumed. 
TABLE 1 
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o-XYLENE OXIDATION RETARDING EFFECT OF 
Cu, Fe and Ni 
Metals Concentration 
in parts Mole % Theoretical 
per 1 .times. 10.sup.6 parts of Co 
Oxygen Consumed 
______________________________________ 
Cu, 80 ppm 113 
Cu, 800 ppm 67 
Cu, 8000 ppm 65 
Cu, 9100 ppm 42 
Cu, 80,000 ppm 28 
Fe, 500 ppm 116 
Fe, 5000 ppm 103 
Fe, 50,000 ppm 70 
Ni, 6700 ppm 112 
Ni, 67,000 ppm 68 
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Thus for such neat oxidation of liquid o-xylene with air there should not 
be present in the original or recycle catalyst an amount of copper in the 
range of from 80 up to 8000, or of iron in the range of from 5000 up to 
50,000, and/or of nickel in the range of from 6700 up to 67,000 weight 
parts per million weight parts of cobalt. 
The problem left by the prior art is how to remove the nickel to the safe 
level of about 6700 ppm, that is, a nickel content of not more than 6700 
ppm of the cobalt and to remove the copper to a safe level of about 80 
ppm, that is, not more than 100 ppm, preferably not more than 80 ppm, of 
the cobalt.

STATEMENT OF THE INVENTION 
We have discovered that an aqueous solution containing cobalt, manganese, 
copper, nickel, and iron having more copper than 80 to 100 ppm, more 
nickel than 6700 ppm and more iron than 500 to 5000 ppm of the cobalt can 
be treated to remove nickel as well as copper and iron. Said method of 
removing nickel comprises adding to said solution, before or after copper 
and iron removal, of at least 2.5 gram moles and up to 10 gram moles of 
nitrilotriacetate (NTA) for each 1.0 gram atom of total contaminant metals 
present in solution and thereafter precipitating cobalt and manganese as 
their carbonates. 
While copper can be precipitated as its sulfide from the aqueous solution 
by the addition thereto of hydrogen sulfide and separating the aqueous 
solution from the solid copper sulfide precipitate, it is preferred to 
remove the copper by the technique of copending patent application Ser. 
No. 3,366, filed Jan. 15, 1979. Said preferred copper removal technique 
comprises contacting the aqueous solution containing copper, nickel, iron, 
cobalt and manganese with particulated iron (e.g., iron filings or iron 
cuttings) at a solution pH between 5 and 7, preferably at a pH of 6, for 
not more than 60 minutes. The preferred time of contact between the 
aqueous solution and particulated iron is from 10 up to 60 minutes. In 30 
to 60 minutes the copper concentration can be decreased below detectable 
limits (detectable limit is 1.0 ppm) by copper plating out on the iron. 
Other metals can, of course, plate out on the iron but copper plates out 
first. For example cobalt begins to plate out after 1200 minutes and from 
1275 minutes and thereafter a significant decrease in cobalt and nickel 
concentrations by their plating out also occurs. An increase in the iron 
content of the solution appears after sixty minutes of contact. Such 
preferred removal of copper from aqueous solution can be accomplished 
before or after, preferably before, the removal of nickel according to the 
present inventive use of NTA. 
The iron content of the aqueous solution is, of course, precipitated with 
the cobalt and manganese when they are converted to their carbonates. 
However, the iron can be readily separated from cobalt and manganese by 
the technique of British Pat. No. 1,413,488. Also the iron content of the 
aqueous solution can be suitably decreased, before such carbonate 
precipitation, by the techniques of U.S. Pat. No. 3,673,154 by adding 
acetic acid or anhydride to the solution until a pH of above 3 (e.g., a pH 
from 3.1 to 4.5), heating the pH adjusted mixture to a temperature of 
93.degree. C. for 15 minutes and then removing by filtration the 
precipitated form of iron. 
The nickel removal concept of this invention is based on the addition to 
the aqueous solution containing ions of cobalt, manganese and nickel and 
possibly iron and copper of a ligand which would react preferentially with 
one or more of the contaminant metals (copper, iron, and/or nickel) 
keeping the contaminant metal soluble when the cobalt and manganese are 
precipitated as their carbonates. The ability of a given metal to react 
with a given ligand in water is governed by the magnitude of its 
equilibrium constant between the ligand and metal (usually called 
"stability constants"). The stability constants for hundreds of such 
combinations of metals and ligands are found in the publication "Stability 
Constants", Special Publication Nos. 17 and 25 of the Chemical Society, 
London edited by Lars G. Sillen and Arthur E. Martell. One will find in 
general that the stability constants of a given ligand with the metals of 
interest generally have the relative magnitudes Fe Cu Ni Co Mn. As a 
result, a given ligand when placed in water will preferentially react with 
all the impurity metals rather than cobalt and manganese if the 
concentrations of all the metals are the same. By use of such constants in 
thermodynamic calculations one finds that in most cases a given ligand 
prefers the impurity metals when the impurity metal concentrations are 
considerably less than that of cobalt and manganese as one normally finds 
in solutions of interest. Not only must the given ligand preferentially 
react with the impurity metals (iron, copper and/or nickel), but it must 
solubilize the metal somewhat during addition of carbonate. One finds by 
thermodynamic calculations that the ability for a ligand to solubilize a 
metal during carbonate addition is directly proportional to the magnitude 
of the stability constant of the ligand and metal. One can conclude for 
example that the relative solubilizing effect increases with the ligands 
oxalate (hereafter "OXA"), nitrilotriacetate (hereafter "NTA"), and 
ethylenediaminetetraacetate (hereafter "EDTA"). 
From the tables to follow it is seen that oxalate (at the amounts of OXA 
added) does selectively solubilize the impurity metals but does not 
solubilize the impurity metals enough. EDTA does solubilize the metals but 
does not do so selectively. NTA has both the selectivity toward nickel and 
the solubilizing ability toward nickel to make it useful to separate 
nickel from the other metals. 
In each of the following four examples (three comparative examples and one 
illustrative example) the starting aqueous solution was the same and had 
the following composition wherein the concentrations of metals other than 
cobalt are also shown in weight ratios per 1.times.10.sup.6 weight parts 
of cobalt. 
TABLE II 
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Cobalt 2380 ppm 1 .times. 10.sup.6 
Chromium 19 ppm 8 .times. 10.sup.3 
Copper 10 ppm 4.2 .times. 10.sup.3 
Iron 87 ppm 37 .times. 10.sup.3 
Manganese 4100 ppm 1.7 .times. 10.sup.6 
Nickel 36 ppm 15 .times. 10.sup.3 
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TABLE III 
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CARBONATE PRECIPITATION WITH NO SOLUBILIZING 
LIGAND 5.4 MOLES CARBONATE PER GRAM MOLE 
CO AND MN 
Metal % Metals Remaining in Solution 
______________________________________ 
Cobalt 0.3 
Chromium 7.4 
Copper 3.5 
Iron 1.2 
Manganese 0.03 
Nickel 2.4 
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TABLE IV 
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CARBONATE PRECIPITATION 5.3 MOLES CO.sub.3 PER 
GRAM ATOM COBALT AND MANGANESE 
2.8 Moles NTA/gram atom 
5.7 Moles NTA/gram atom 
total gram atoms total gram atoms 
Contaminant Metals Contaminant Metals 
______________________________________ 
Metal Remaining in Soln., % 
Metal Remaining in Soln., 
Cobalt 10.3 Cobalt 21 
Chromium 2.6 Chromium 0.3 
Copper 27 Copper 30 
Iron 0.6 Iron 1.3 
Manganese 0.04 Manganese 0.07 
Nickel 83 Nickel 92 
Composition of Precipitate 
Composition of Precipitate 
Basis: Cobalt = 1 .times. 10.sup.6 
Basis: Cobalt =1.0 .times. 10.sup.6 
Chromium 13,000 Chromium 15,000 
Copper 3,300 Copper 3,800 
Iron 56,000 Iron 51,000 
Manganese 2,000,000 Manganese 2,260,000 
Nickel 6,400 Nickel 6,800 
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TABLE V 
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CARBONATE PRECIPITATION 5.4 MOLES CO.sub.3 PER 
GRAM ATOM COBALT AND MANGANESE 
2.7 Moles OXA/gram atom 
5.5 Moles OXA/gram atom 
total gram atoms total gram atoms 
Contaminant Metals Contaminant Metals 
Metal Remaining in Soln., % 
Metal Remaining in Soln., % 
______________________________________ 
Cobalt 0.3 Cobalt 0.9 
Chromium 2.6 Chromium 6.1 
Copper 8.2 Copper 5.0 
Iron 0.8 Iron 1.0 
Manganese 0.02 Manganese 0.04 
Nickel 7.7 Nickel 14 
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TABLE VI 
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CARBONATE PRECIPITATION 5.7 MOLES 
CO.sub.3 /GRAM ATOM COBALT AND MANGANESE 
5.7 Moles EDTA/gram atom total 
gram atoms of Contaminant Metals 
Metals in Precipitate: 
Metals Remaining in Soln., % 
Co = 1 .times. 10.sup.6 
______________________________________ 
Cobalt 29.6 Chromium 17.00 
Chromium 4.9 Copper 4,900 
Copper 13 Iron 71,000 
Iron 1.0 Manganese 2,600,000 
Manganese 1.1 Nickel 23,000 
Nickel 12.1 
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From the foregoing it is seen that neither EDTA nor oxalate have the 
selective solubilizing needed for a useful ligand for decreasing 
contaminant metal, especially nickel, when precipitating cobalt and 
manganese as their carbonates.