Process for the preparation of salts of iron amino and hydroxy carboxylic acid complexes

A process of preparing iron chelates of amino and hydroxy carboxylic acids, comprising reacting an oxide of iron with an amino or hydroxy carboxylic acid and a base in the presence of ferrous ion or metallic iron as a catalyst. Additional base is added to the resulting chelant and oxidation may be carried out to convert any ferrous ion to ferric.

BACKGROUND OF THE INVENTION 
The present invention generally relates to salts of complexes of 
aminocarboxylic and hydroxycarboxylic acids, and in particular, to the 
process of preparing salts of iron complexes of aminocarboxylic and 
hydroxycarboxylic acids, such 
iron chelates have numerous applications, such as in agriculture, in 
photographic processing, and as food additives. 
One conventional route to ferric ammonium EDTA complex is the reaction of 
diammonium EDTA solution with sponge iron at a temperature of 60.degree. 
C. or higher for approximately 10 hours, thereby producing ferrous EDTA. 
The ferrous EDTA is then oxidized to ferric EDTA by air sparging or many 
hours. However, the production of the ferrous EDTA intermediate releases 
H.sub.2, which is potentially explosive. The evolution of H.sub.2 also 
often causes a foaming problem which limits the batch size. The sponge 
iron contains traces of sulfur and phosphorus; during the reaction with 
EDTA these are emitted as phosphine (PH.sub.3) and hydrogen sulfide 
(H.sub.2 S) which generate obnoxious odors. In addition, the total 
processing time is in excess of 24 hours. 
U.S. Pat. No. 4,558,145 to Smith et al discloses a process for preparing 5% 
iron solutions of the ferric chelate of hydroxyethlyenediaminetriacetic 
acid from the trisodium salt of that acid, nitric acid and metallic iron. 
The ferrous chelate thus produced is then converted to the ferric chelate 
by air oxidation. 
Japanese Kokai 53-35929 discloses a process for preparing ferric chelates 
wherein a mixture of 5-74 weight % Fe.sub.3 O.sub.4, 25-95 weight % iron, 
and depending on the circumstances, 0-60 weight % of a water soluble iron 
salt are made to react with a chelating agent or its alkaline salts in an 
aqueous medium not above room temperature. The ferrous chelate is 
simultaneously or subsequently oxidized to ferric by air sparging. This 
process has the advantage that little hydrogen is evolved; however, all 
the iron complex produced from the Fe.sub.3 O.sub.4 /Fe.degree. reaction 
is ferrous iron and thus all must be oxidized to ferric iron. 
Japanese Kokai 59-167595 discloses a process for Preparing ferric chelates 
wherein the rate of reaction of the chelating agent with hydrated iron 
oxide is accelerated by the addition of a reducing agent such as hydrazine 
or sodium hydrosulfite. 
U.S. Pat. No. 3,767,689 to Donovan et. al. discloses a process for 
preparing water-soluble ammonium salts of ferric aminocarboxylic acid 
complexes by heating iron oxide with an aminocarboxylic acid or a 
partially neutralized aminocarboxylic acid in an aqueous medium, and 
neutralizing the resulting ferric complex by reacting it with a base, such 
as ammonium, sodium, or potassium hydroxide. 
U.S. Pat. No. 4,364,871 to Svatek et. al. discloses a process for preparing 
aminopolycarboxylic acid chelates of iron by reacting ammonia and the 
aminopolycarboxylic acid in a mole ratio of about 1-1.5:1 (NH.sub.3 
aminopolycarboxylic acid) in the presence of iron oxide. After the iron 
oxide is completely reacted with the chelant, the mixture is cooled and 
sufficient ammonia is introduced to dissolve and maintain the iron chelate 
in solution. The reaction mixture is cooled, and contacted with air to 
oxidize any remaining ferrous to ferric. 
These processes suffer from various drawbacks, such as the necessity of 
elevated reaction temperatures which results in the deleterious 
decomposition of the aminopolycarboxylic acid, the necessity to use 
expensive synthetic iron oxides and/or reducing agents to achieve 
acceptable reaction temperatures and times, the generation of all iron in 
the Fe(II) oxidation state, or the evolution of explosive hydrogen. 
SUMMARY OF THE INVENTION 
The problems of the prior art have been overcome by the present invention, 
which provides a process for preparing salts of iron complexes of 
aminocarboxylic and hydroxycarboxylic acids. Generally, the present 
invention involves reaction of the free acid, or the ammonium, sodium, 
lithium, or potassium salts or partial salts of an aminocarboxylic or 
hydroxycarboxylic acid, with an oxide of iron such as magnetite (Fe.sub.3 
O.sub.4) or hydrated iron oxide [Fe(OH)0] in the presence of a catalyst, 
followed by the addition of more base and, optionally, oxidation to 
convert any Fe(II) to Fe(III). The reaction proceeds at lower temperatures 
and more rapidly than prior art processes, thereby minimizing product 
decomposition and reaction time. No phosphine or hydrogen sulfide is 
emitted. In addition, no hydrogen gas is produced; therefore, no reduction 
of Fe(III) to Fe(II) occurs from hydrogen gas. Prior art processes which 
employ magnetite as the primary iron source generally require the use of 
expensive synthetic magnetite. In contrast, for some ligands, such as 
EDTA, the instant process can be practiced with inexpensive magnetite from 
natural sources. 
In a preferred embodiment of the present invention, the reaction of 
magnetite and the salt or partial salt of an aminocarboxylic acid (such as 
EDTAH.sub.3.5 (NH.sub.4).sub.0.5) or a hydroxycarboxylic acid (such as 
citric acid monoammonium salt) is catalyzed by the addition of a soluble 
ferrous salt, followed by addition of more base (NH.sub.3 in the above 
instance) and then air oxidation. 
In another embodiment of the present invention, the reaction of the iron 
oxide and the free acid or salt or partial salt of an aminocarboxylic or 
hydroxy carboxylic acid is catalyzed by the addition of trace amounts of 
finally divided metallic iron. 
In a further embodiment of the present invention, the reaction of the iron 
oxide and the free acid or salt or partial salt of an aminocarboxylic or 
hydroxycarboxylic acid is catalyzed by the addition of trace amounts of 
the salt of the iron (II) complex being produced.

DETAILED DESCRIPTION OF THE INVENTION 
Suitable aminocarboxylic acids that are useful in the present invention as 
the chelant moiety are those which are capable of chelating iron, 
including nitrilotriacetic acid (NTA); iminodiacetic acid (IDA); 
1,2-propylene-diaminetetraacetic acid (PDTA); 
1,3-propanediaminetetraacetic acid (1,3-PDTA); ethylenediaminetetraacetic 
acid (EDTA); N-methyl, ethyl, propyl and butyl iminodiacetic acids; 
triethylenetriaminehexaacetic acid; 
ethyleneglycolbis(aminoethylether)tetraacetic acid; 
cyclohexane-1,2-diaminotetraacetic acid; diamino-2-propanoltetracetic 
acid; hydroxyethyliminodiacetic acid; dihydroxyethylglycine; ethanol 
diglycine; ethylenediamine ortho hydroxyphenylacetic acid; 
N-hydroxyethylethylenediaminetriacetic acid (HEDTA); and 
diethylenetriaminepentaacetic acid (DTPA). For IDA; N-methyl, ethyl, 
propyl and butyl iminodiacetic acids and for dihydroxyglycine and ethanol 
diglycine, the iron chelate will have a mole ratio of Fe:ligand of about 
1:2. For purposes of simplicity, EDTA will be used hereinafter as 
illustrative of the aminocarboxylic acid, although it should be understood 
that other aminocarboxylic acids can be used. 
Suitable hydroxy carboxylic acids that may be useful in the present 
invention as the chelant moiety are those which are capable of chelating 
iron, including citric acid, tartaric acid, lactic acid and gluconic acid. 
Suitable iron oxides include magnetite and alpha and gamma hydrated iron 
oxides. The gamma hydrated iron oxide (lepidocrocite) is about twice as 
reactive as the alpha form (goethite). The iron in the hydrated oxides is 
in the 3.sup.30 oxidation state, therefore their use would generally 
eliminate the oxidation step. However, cost and availability (especially 
of the gamma form) may dictate that other oxides should be used. Hematite 
or red iron oxide is the least reactive iron oxide, and seems to result in 
excessive decomposition of the aminocarboxylic acid. Magnetite is the most 
reactive and the least expensive, and is therefore the preferred oxide. 
Synthetic magnetites are the purest form, but are relatively expensive. 
Some natural magnetites can be used where levels of impurities such as 
heavy metals, silica, and alumina, are acceptable. The presence of alumina 
and silica can cause severe filtration problems, rendering natural 
magnetites that are heavily contaminated with silica and alumina 
inappropriate. Some have surface properties which render them inert in the 
present process. The preferred natural magnetite is Tamms Magnetite, 
available from Tamms Industries. The preferred synthetic magnetite is 
Mapico Black. Of these, Tamms Magnetite is preferred due to its lower 
cost. 
Where synthetic magnetites are used, the reaction temperature employed with 
EDTA should be about 40.degree. C., preferably about 40-60.degree. C., 
most preferably about 40-45.degree. C. 
Natural magnetites require higher reaction temperatures of at least about 
60.degree. C. with EDTA, preferably about 60.degree. C.-80.degree. C., 
most preferably about 60.degree.-65.degree. C. 
Ligands other than EDTA may require higher reaction temperatures with both 
synthetic and natural magnetites, which one skilled in the art readily 
will be able to determine. 
A mixture of magnetite and sponge iron (above catalytic amounts) also could 
be used, but it produces a slurry of only the ferrous chelate. Longer 
oxidation is then required to convert Fe(II) to Fe(III), which , in turn, 
would lead to greater decomposition of the product. The all-magnetite 
process produces a slurry in which only 20-25% of the total iron is in the 
ferrous state; the balance is already ferric. As a result, less time is 
required for air oxidation and decomposition of product is mitigated. 
The selection of a suitable base depends upon the particular complex salt 
that is desired. Ferric ammonium EDTA is prepared by employing NH.sub.3 
(such as 28% NH.sub.3) in a mole ratio of NH.sub.3 : EDTAH.sub.4 of no 
greater than about 2.0:1.0, preferably about 0.1-1.0:1.0, most preferably 
about 0.5:1.0. Similarly, ferric sodium EDTA and ferric potassium EDTA can 
be prepared using NaOH and KOH respectively, as the base, in similar 
amounts. Where the starting acid is in the form of its salt or partial 
salt, the neutralizing base is one whose cation is also the salifying ion. 
The rates of reaction of the oxide with the acid are pH and temperature 
dependent. In the absence of a catalyst, reaction temperatures of about 
90.degree.-110.degree. C. are necessary. In the process of the present 
invention, rates of reaction can be increased in the order of about 2-10 
fold by the use of a trace amount of a soluble (.e(II) salt, a trace 
amount of Fe.degree., or, a trace amount of the salt of the iron (II) 
complex being produced, to catalyze the reaction of the oxide with the 
acid. As the Fe(II) salt, any ferrous salt which will dissolve in the 
reaction mixture and whose anion is not deleterious to the process or end 
application of the product can be used. The preferred inorganic salt is 
FeSO.sub.4.xH.sub.2 O because it is inexpensive, very soluble in the 
reaction mixture, and sulfate causes no problems in the end application. 
Other suitable inorganic salts include ferrous carbonate, chloride, 
bromide and nitrate. The preferred organic salt is the ferrous chelate of 
the ligand being produced. Other suitable organic salts include ferrous 
gluconate, citrate and glycolate. The salt should be used in a trace 
amount, which for FeSO.sub.4.xH.sub.2 O (and EDTA as the ligand) is a 
contained iron mole ratio of FeSO.sub.4.xH.sub.2 O:Fe.sub.3 O.sub.4 of 
about 0.002-0.05:1, preferably about 0.01-0.02:1, an equivalent amount for 
the other salts and about 0.0007-0.017:1 for Fe.degree.. 
In the preparation of ferric ammonium EDTA, for example, after the 
catalyzed reaction of the iron oxide with EDTAH.sub.3.5 
(NH.sub.4).sub.0.5, additional base (NH.sub.3) is added. The mono ammonium 
salt, EDTANH.sub.4 Fe, can be produced by limiting the total NH.sub.3 
charge to 1.0 moles NH.sub.3 : 1.0 moles EDTAH.sub.4 Ferric ammonium EDTA 
complex, EDTA(NH.sub.4).sub.2 FeOH is formed where the total charge of 
NH.sub.3 is higher. A suitable amount of NH.sub.3 added after the 
catalyzed reaction is about 1.4-2.0 moles per mole of EDTAH.sub.4. 
Oxidation of any Fe(II) to Fe(III) can be accomplished by any suitable 
means, such as by sparging with air or oxygen. Air oxidation is preferred. 
At an oxidation temperature above about 40.degree. C., decomposition of 
the amino or hydroxy carboxylic acid moiety increases rapidly. Preferably, 
oxidation is carried out in a temperature range of about 
25.degree.-35.degree. C. 
The preferred order of addition of reactants is as follows. Water, the iron 
oxide, and base are mixed prior to the addition of the acid. (If the base 
is added after mixing water, the acid and the iron oxide, the subsequent 
reaction may become sluggish and the reaction incomplete, especially with 
natural magnetites). When the temperature is equilibrated, the catalyst is 
then added. Thus, in the Preferred process of preparing EDTA(NH.sub.4)Fe, 
water, ammonia, and synthetic magnetite are mixed, EDTA acid is added and 
the resulting slurry is equilibrated at about 40.degree. C. Ferrous 
sulfate catalyst is then added, so that the mole ratios (Fe compounds in 
terms of contained Fe) of NH.sub.3 :EDTA:magnetite:FeSO.sub.4 are about 
0.530:1.06:1.00:0.01. After the chelation reaction is complete, 
approximately 25% of the total iron is as Fe(II) and approximately 75% is 
as Fe(III). The slurry is neutralized with NH.sub.3 to a PH of about 5-7 
and is air-oxidized at a temperature of about 30.degree. C. The Fe(III) NH 
EDTA solution is then converted to the complex (EDTA(NH 2FeOH) by the 
addition of more NH.sub.3, diluted to final concentration, and filtered to 
remove any small amount of insoluble residue derived from the magnetite. 
To produce solid products, crystallization or a total drying process such 
as spray drying can be used to produce the mono ammonium, sodium or 
potassium salt of the Fe(III) chelate. 
The present invention will now be illustrated by the following non-limiting 
examples. 
EXAMPLE 1 
In order to demonstrate the catalytic effect of the addition of ferrous 
ion, a series of runs was made using NH.sub.3 EDTA mole ratios of 0.5 and 
1.0:1.0 at 40.degree. C. and 60.degree. C. The mole ratio of EDTA:TOTAL Fe 
was 1.10:1.0. The mole ratio of Fe(II):Fe in magnetite was 0.01:1.0. The 
magnetite used was Mapico Black. In runs 1, 7 and 8, the pH was raised to 
approximately 6 with NH.sub.3 prior to air sparging. The runs summarized 
in Table I. 
At the NH.sub.3 : EDTAH.sub.4 mole ratio of 0.5:1.0 and at 40.degree. C., 
the time required for complete dissolution of the magnetite was 
approximately 45 minutes Run no. 4 demonstrates the catalytic effect of 
Fe(II). In that run, the EDTAH.sub.4 /NH.sub.3 mixture was equilibrated at 
40.degree. C. and held at that temperature for 3 hours, and no reaction 
occurred. Upon adding FeSO.sub.4, the reaction began immediately. Run no. 
3 shows that the reaction proceeds without the need for Fe(II) catalyst if 
the temperature is raised to 60.degree. C. 
At the NH.sub.3 : EDTAH.sub.4 mole ratio of 1.0:1.0, the reaction is much 
slower than at the lower ratio. In Run no. 5, the NH.sub.3/ EDTAH.sub.4/ 
magnetite mixture stirred at 40.degree. C. with Fe(II) catalyst present, 
and no reaction occurred until the temperature was raised to 60.degree. C. 
Similarly, in run no. 6, the mixture without catalyst stirred at 
60.degree. C. for 30 minutes with no reaction occurring, and, upon 
addition of Fe(II), the reaction started immediately. 
TABLE 1 
__________________________________________________________________________ 
Mole Ratio Time to Comments 
Run 
NH3:1.00 
Reation 
Dissolve All 
Air Sparge For those lots which were air sparged, 
the pH was 
No. 
EDTAH4 
Temp. .degree.C. 
Fe (Min) 
Temp., .degree.C. 
Minutes 
raised 10.sup.-6 with NH3 before 
sparging began. 
__________________________________________________________________________ 
1 0.5 40 45 40 90 
2 0.5 40 45 0 
3 0.5 60 .about.30 0 No Fe(II) added 
4 0.5 40 45-60 0 Held 3 hr @ 40.degree. C. before 
Fe(II) added - no reaction 
until Fe(II) was added 
5 1.0 40/60 0 No reaction after 60 min with Fe(II) 
present - 
heated to 60.degree. C. - reaction 
began immediately 
6 1.0 60 60 Held 30 min @ 60.degree. C. before 
Fe(II) added - no reaction 
until Fe(II) added 
7* 
1.0 60 60 30 90 Repeat of No. 6 but Fe(II) added as 
soon as T reached 
60.degree. C. - reacted immediately. 
This lot was air sparged. 
8* 
0.5 40 90 30 90 
__________________________________________________________________________ 
Analytical Data 
Ferrous Iron Ferric Iron 
Free EDTA 
No. 
1st Final 
1st Final 
1st Final 
__________________________________________________________________________ 
1 1.48% 
0.11% 
5.89% 
5.56% 
n.a. 
n.a. 
2 1.73% 
n.a. 
5.89% 
n.a. 
n.a. 
n.a. 
3 1.58% 
n.a. 
5.76% 
n.a. 
n.a. 
n.a. 
4 2.07% 5.35% 
5 3.48% 
6 4.17% 
7* 
2.09% 
0.22% 
7.11% 
7.48% 
4.54% 
n.a. 
8* 
1.75% 
0.28% 
6.98% 
6.95% 
6.12% 
n.a. 
__________________________________________________________________________ 
*Water charge reduced to give 7% total Fe 
Note: 1st = sample at end of dissolution or just before sparge 
EXAMPLE 2 
A survey of the relative reaction rates of various ligands with magnetite 
as a function of mole ratio of base:ligand (or pH) was run. Water, ligand 
in the free acid form, ammonia solution, and synthetic magnetite were 
weighed into 20 ml pressure tubes. The pH was measured, then ferrous 
sulfate was weighed into some of the tubes and the tubes were sealed with 
a threaded teflon stopper fitted with an O-ring seal. The tubes were 
mounted on a rotating rack inside of a forced draft oven. The oven was 
turned on and the rack was rotated. When the oven temperature reached 
70.degree. C. a timer was started. The tubes were periodically examined. 
By bringing the tubes between the poles of a heavy horseshoe magnet, it 
was possible to determine when the magnetite had dissolved. 
These experiments are summarized in Table II. These data show that the 
ferrous catalyzed magnetite/ligand reaction is generally applicable to a 
wide variety of ligands, but that the optimum conditions are specific for 
each, and the rates of reaction vary widely. The generally operable pH 
range is about 2.2-4.2. There may be other ligands which may have optimum 
pH ranges somewhat outside of this range; those skilled in the art readily 
will be able to determine the optimum conditions for ligands other than 
those included in Table II. 
EXAMPLE 3 
The general procedure of Example 2 was used but the ligand was 
Hamp-ol.RTM.-120, a commercial 41.3% solution of 
hydroxyethylenediaminetriacetic acid (HEDTA) trisodium salt. The ligand 
was weighed into a series of tubes. 
TABLE II 
__________________________________________________________________________ 
SUMMARY OF LIGAND/MAGNETITE SCREENING PROGRAM 
__________________________________________________________________________ 
EDTA 
LIGAND A E* B F* ** 
C G* D H* 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.00 0.00 
0.50 
0.50 1.00 1.00 
1.50 1.50 
Ligand:Fe in magnetite 
1.10 1.12 
1.10 
1.12 1.10 1.12 
1.10 1.12 
Fe in magnetite 
1.00 1.00 
1.00 
1.00 1.00 1.00 
1.00 1.00 
FeSO4:Fe 0.000 
0.020 
0.000 
0.020 
0.000 
0.020 
0.000 
0.020 
in magnetite 
MILLI-MOLES Ligand 19.25 
19.25 
19.25 
19.65 
19.25 
19.64 
19.25 
19.64 
NH3 0.00 0.00 
9.63 
9.82 19.25 
19.64 
28.88 
29.45 
Fe in magnetite 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
FeSO4 0 0.35 
0 0.35 0 0.35 
0 0.35 
TOTAL GRAMS 15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
pH initial 3.10 3.13 
4.19 
4.18 4.41 4.38 
4.56 4.55 
final 0.73 0.78 
2.32 
2.40 2.97 2.98 
5.77 5.77 
MINUTES TO DISSOLVE .about.80 
.about.60 
.about.80 
&gt;5 &lt; 10 
.about.80 
.about.20 
&gt;180 &gt;180 
__________________________________________________________________________ 
DTPA 
LIGAND A B F* ** 
C G* D H* E I* 
__________________________________________________________________________ 
MOLE RATIO 
NH3:Ligand 
0.00 0.50 0.50 1.00 1.00 1.50 1.50 
2.00 2.00 
Ligand:Fe in 
1.10 1.10 1.12 1.10 1.12 1.10 1.12 
1.10 1.12 
magnetite 
Fe in magnetite 
1.00 1.00 1.00 1.00 1.00 1.00 1.00 
1.00 1.00 
FeSO4:Fe in 
0.000 
0.000 
0.020 
0.000 0.020 
0.000 0.020 
0.000 
0.020 
magnetite 
MILLI- Ligand 19.25 
19.25 
19.64 
19.25 19.64 
19.25 19.64 
19.25 
19.64 
MOLES NH3 0.00 9.63 9.82 19.25 19.64 
28.88 29.45 
38.50 
39.27 
Fe in magnetite 
17.50 
17.50 
17.50 
17.50 17.50 
17.50 17.50 
17.50 
17.50 
FeSO4 0 0 0.35 0 0.35 0 0.35 
0 0.35 
TOTAL 15.00 
15.00 
15.00 
15.00 15.00 
15.00 15.00 
15.00 
15.00 
GRAMS 
pH initial 2.38 3.55 3.60 3.83 3.87 4.05 4.08 
4.31 4.37 
final 2.12 2.16 2.05 2.68 2.67 3.58 3.56 
4.62 4.70 
MINUTES 30 90 5 to 10 
.about.135 
15 180 30 330 .about.90 
TO 
DISSOLVE 
__________________________________________________________________________ 
HEDTA 
LIGAND A E* ** 
B F* ** 
C G* D H* 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.00 0.00 
0.25 0.25 
0.50 0.50 
1.00 1.00 
Ligand:Fe in magnetite 
1.10 1.12 
1.10 1.12 
1.10 1.12 
1.10 1.12 
Fe in magnetite 
1.00 1.00 
1.00 1.00 
1.00 1.00 
1.00 1.00 
FeSO4:Fe in magnetite 
0.000 
0.020 
0.000 
0.020 
0.000 
0.020 
0.000 
0.020 
MILLI-MOLES Ligand 19.25 
19.64 
19.25 
19.64 
19.25 
19.64 
19.25 
19.64 
NH3 0.00 0.00 
4.81 4.91 
9.63 9.82 
19.25 
19.64 
Fe in magnetite 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
FeSO4 0 0.35 
0 0.35 
0 0.35 
0 0.35 
TOTAL GRAMS 15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
pH initial 2.20 2.22 
3.26 3.29 
3.65 3.68 
4.30 4.28 
final 2.11 2.09 
2.58 2.55 
3.09 3.04 2.11 
MINUTES TO DISSOLVE 50 15 80 15 .about.120 
30 &gt;&gt;240 
.about.150 
__________________________________________________________________________ 
1,3-PDTA 
LIGAND A C* ** 
B D* 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.50 0.50 
1.00 1.00 
Ligand:Fe in magnetite 
1.10 1.12 
1.10 1.12 
Fe in magnetite 
1.00 1.00 
1.00 1.00 
FeSO4:Fe in magnetite 
0.000 
0.020 
0.000 
0.020 
MILLI-MOLES Ligand 19.25 
19.44 
19.25 
19.44 
NH3 9.63 9.72 
19.25 
19.44 
Fe in magnetite 
17.50 
17.50 
17.50 
17.50 
FeSO4 0 0.35 
0 0.35 
TOTAL GRAMS 15.00 
15.00 
15.00 
15.00 
pH initial 3.16 3.17 
3.44 3.45 
final 2.58 2.65 
3.37 3.32 
MINUTES TO DISSOLVE &gt;&gt;300 
.about.150 
&gt;300 &gt;120 &lt; 180 
__________________________________________________________________________ 
NTA 
LIGAND A E* ** 
B F* C G* D H* 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.00 0.00 0.25 0.25 
0.50 0.50 
1.00 
1.00 
Ligand:Fe in magnetite 
1.10 1.12 1.10 1.12 
1.10 1.12 
1.10 
1.12 
Fe in magnetite 
1.00 1.00 1.00 1.00 
1.00 1.00 
1.00 
1.00 
FeSO4:Fe in magnetite 
0.000 0.020 
0.000 
0.020 
0.000 
0.020 
0.000 
0.020 
MILLI-MOLES Ligand 19.25 19.64 
19.25 
19.64 
19.25 
19.64 
19.25 
19.64 
NH3 0.00 0.00 4.81 4.91 
9.63 9.82 
19.25 
19.64 
Fe in magnetite 
17.50 17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
17.50 
FeSO4 0 0.35 0 0.35 
0 0.35 
0 0.35 
TOTAL GRAMS 15.00 15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
15.00 
pH initial 2.34 2.38 3.17 3.15 
3.30 3.29 
3.49 
3.47 
final 2.51 2.74 
4.26 4.09 
4.52 
4.60 
MINUTES TO DISSOLVE &gt;&gt;180 60 to 75 
&gt;&gt;180 
.about.120 
.about.90 
.about.90 
&gt;150 
.about.150 
__________________________________________________________________________ 
CITRIC ACID 
LIGAND I M* J N* K O* L P* 
__________________________________________________________________________ 
** 
MOLE RATIO 
NH3:Ligand 0.00 0.00 0.25 0.25 0.50 0.50 1.00 1.00 
Ligand:Fe in magnetite 
1.10 1.12 1.10 1.12 1.10 1.12 1.10 1.12 
Fe in magnetite 
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 
FeSO4:Fe in magnetite 
0.000 0.020 
0.000 
0.020 0.000 
0.020 
0.000 
0.020 
MILLI-MOLES 
Ligand 19.25 19.64 
19.25 
19.64 19.25 
19.64 
19.25 
19.64 
NH3 0.00 0.00 4.81 4.91 9.63 9.82 19.25 
19.64 
Fe in magnetite 
17.50 17.50 
17.50 
17.50 17.50 
17.50 
17.50 
17.50 
FeSO4 0 0.35 0 0.35 0 0.35 0 0.35 
TOTAL GRAMS 15.00 15.00 
15.00 
15.00 15.00 
15.00 
15.00 
15.00 
pH initial 1.19 1.20 2.17 2.22 2.63 2.62 3.36 3.33 
final 3.60 3.23 
MINUTES TO &gt;&gt;180 &gt;&gt;180 
&gt;&gt;180 
&gt;&gt;180 &gt;&gt;180 
&gt;&gt;180 
60 15 to 50 
DISSOLVE 
__________________________________________________________________________ 
Notes: 
Iron compounds are expressed in terms of contained Fe 
Magnetite is Mapico Black, a synthetic magnetite 
Temperature is 70.degree. C. 
*designates mixtures containing ferrous sulfate 
**designates most rapid reaction of the set 
Nitric acid was added to each to adjust each to various pH values. 
Synthetic magnetite was then added. Ferrous sulfate was added to selected 
tubes. The tubes were then rotated in the oven at 70.degree. C. and 
observed for dissolution of the magnetite as in Example 2 The results are 
summarized in Table III. 
The optimum pH (2.8) corresponded approximately to neutralization of the 
contained HEDTA to the free acid. Without the addition of ferrous sulfate, 
the time required to dissolve the magnetite was 60-105 minutes. With 
ferrous sulfate present, the time needed was 15-30 minutes at an 
HN03:ligand mole ratio of 3.07:1. 
EXAMPLE 4 
The general procedure of Example 2 was used. Optimum ammonia:ligand mole 
ratios for various ligands were used. Instead of ferrous sulfate, the 
catalyst was finely divided metallic iron, commonly known as sponge iron. 
The amount of sponge iron used was chemically equivalent to the amount of 
ferrous sulfate used in Example 2. The results are summarized in Table IV. 
The results demonstrate that sponge iron is an effective catalyst. 
EXAMPLE 5 
To a 1 liter stirred vessel was added 315 g of water, 66.7 g of natural 
magnetite (0.850 moles of contained Fe), and 25.6 g of 29.4% NH.sub.3 
(0.442 moles). To this mixture was added 258.4 g of EDTA acid (0.884 
moles). 
TABLE III 
__________________________________________________________________________ 
SUMMARY OF [LIGAND(Na)x + HNO3] MAGNETITE SCREENING PROGRAM 
Hamp-Ol 120 + HNO3 
LIGAND 48-C 48-D* 
48-A 48-B* 50-E 50-F* ** 
50-G 50-H* 
__________________________________________________________________________ 
MOLE RATIO 
HNO3:Ligand 
2.67 2.68 2.88 2.88 3.08 3.07 3.28 3.28 
Ligand:Fe in 
1.10 1.12 1.10 1.12 1.10 1.12 1.10 1.12 
magnetite 
Fe in magnetite 
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 
FeSO4:Fe in 
0.000 
0.020 
0.000 
0.020 0.000 0.020 0.000 0.020 
magnetite 
MILLI-MOLES 
Ligand 13.20 
13.46 
13.20 
13.46 13.20 13.46 13.20 13.46 
HNO3 35.27 
36.03 
38.00 
38.76 40.62 41.38 43.24 44.11 
Fe in magnetite 
12.00 
12.00 
12.00 
12.00 12.00 12.00 12.00 12.00 
FeSO4 0 0.24 0 0.24 0 0.24 0 0.24 
TOTAL GRAMS 15.19 
15.55 
15.44 
15.80 15.68 16.04 15.92 16.29 
pH initial 3.79 3.80 3.32 3.28 2.87 2.84 2.54 2.52 
final 3.78 3.22 3.10 2.81 2.75 
MINUTES TO &gt;&gt;150 
&gt;&gt;150 
&gt;&gt;150 
&gt;90 &lt; 120 
&gt;90 &lt; &gt;15 &lt; 30 
&gt;60 &lt; 75 
&gt;30 &lt; 45 
DISSOLVE 
% Fe IN FINAL 4.33% 4.27% 4.26% 4.21% 4.20% 
SOLUTION 
(calculated) 
__________________________________________________________________________ 
Notes: 
Iron compounds are expressed in terms of contained Fe 
Magnetite is Mapico Black, a synthetic magnetite 
Temperature is 70.degree. C. 
*designates mixtures containing ferrous sulfate 
**designates most rapid reaction of the set 
TABLE IV 
__________________________________________________________________________ 
SUMMARY OF LIGAND/IRON(0)/MAGNETITE SCREENING PROGRAM 
__________________________________________________________________________ 
EDTA DTPA HEDTA 
LIGAND 1 1A 2 2A 3 3A 
Fe(0) PRESENT 
No Yes No Yes No Yes 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.50 0.50 0.50 0.50 0.00 0.00 
Ligand:Fe in magnetite 
1.10 1.11 1.10 1.11 1.10 1.11 
Fe in magnetite 
1.00 1.00 1.00 1.00 1.00 1.00 
Fe(0):Fe in magnetite 
0.0000 
0.0067 
0.0000 
0.0067 0.0000 
0.0067 
MILLI-MOLES Ligand 19.25 19.38 19.25 19.38 19.25 19.38 
NH3 9.63 9.69 9.63 9.69 0.00 0.00 
Fe in magnetite 
17.50 17.50 17.50 17.50 17.50 17.50 
Sponge Iron 0.0000 
0.1167 
0.0000 
0.1167 0.0000 
0.1167 
TOTAL GRAMS 15.00 15.00 15.00 15.00 15.00 15.00 
pH initial 4.18 4.18 3.59 3.60 2.20 2.20 
final 2.36 2.37 2.98 3.01 2.08 2.11 
MINUTES TO DISSOLVE &gt;60 &lt; 75 
&lt;15 &gt;75 &lt; 90 
&gt;15 &lt; 30 
&gt;30 &lt; 45 
&gt;15 
__________________________________________________________________________ 
&lt; 30 
1,3-PDTA Citric Acid Hamp-Ol 120 + HNO3 
LIGAND 4 4A 6 6A 7 7A 
Fe(0)PRESENT 
No Yes No Yes No Yes 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.50 0.50 1.00 1.00 0.00 0.00 
HNO3:Ligand 0.00 0.00 0.00 0.00 3.08 3.08 
Ligand:Fe in magnetite 
1.10 1.11 1.10 1.11 1.10 1.11 
Fe in magnetite 
1.00 1.00 1.00 1.00 1.00 1.00 
Fe(0):Fe in magnetite 
0.0000 
0.0067 
0.0000 
0.0067 0.0000 
0.0067 
MILLI-MOLES Ligand 19.25 19.38 19.25 19.38 13.20 13.29 
NH3 9.63 9.69 19.25 19.38 0.00 0.00 
HNO3 0.00 0.00 0.00 0.00 40.59 40.86 
Fe in magnetite 
17.50 17.50 17.50 17.50 12.00 12.00 
Sponge Iron 0.0000 
0.1167 
0.0000 
0.1167 0.0000 
0.0800 
TOTAL GRAMS 15.00 15.00 15.00 15.00 15.68 15.78 
pH initial 3.21 3.22 3.40 3.40 2.81 2.81 
final 2.68 3.54 3.54 3.24 3.30 
MINUTES TO DISSOLVE &gt;&gt;210 .about.210 
&gt;75 &lt; 90 
&gt;30 &lt; 45 
.about.105 
&gt;45 
__________________________________________________________________________ 
&lt; 60 
ORDER OF LIGAND REACTION RATES: EDTA &gt; DTPA = HEDTA &gt; Citric Acid &gt; HampO 
120/HNO3 &gt; 1,3PDTA 
Notes: 
Iron compounds are expressed in terms of contained Fe 
Magnetite is Mapico Black, a synthetic magnetite 
Temperature is 70.degree. C. 
The pH of the resulting mixture was 4.0. The slurry was heated to 
63.degree. C. and 2.36 g of FeSO.sub.4.7H.sub.2 O (0.009 moles) was added. 
The temperature was maintained at 60-65.degree. C. After 90 minutes the 
magnetite had dissolved and the pH had dropped to 2.35 and was stable, 
thus indicating that the reaction was finished. A portion of this slurry 
was removed to be used as catalyst for the succeeding run. By analysis, 
the ferrous iron concentration of this sample was 1.77%. 
The previous reaction was repeated except that 26.8 g (0.008 moles of 
Fe(II)) of the retained slurry from the first run was added instead of 
ferrous sulfate. The reaction was finished and the pH was stable within 60 
minutes. 
This example demonstrates that the ferrous chelate of the product being 
produced (in this case, ferrous ammonium EDTA) is an excellent catalyst 
for the system. 
EXAMPLE 6 
The general procedure of Example 2 was used but with gamma hydrated iron 
oxide instead of magnetite as the iron source. The optimum ammonia:ligand 
mole ratios previously found for the various ligands with magnetite were 
used. Tests were run at either 110.degree. C. or 100.degree. C. The 
results are summarized in Table V. 
The data of Table V show that ferrous ion catalyzes the reaction of 
hydrated iron oxide with aminocarboxylic acids, but the rates of reaction 
are slower than with magnetite, and the reaction temperature is 
significantly higher than with magnetite. 
TABLE V 
__________________________________________________________________________ 
SUMMARY OF LIGAND/GAMMA HYDRATED IRON OXIDE SCREENING PROGRAM 
__________________________________________________________________________ 
LIGAND EDTA HEDTA NTA 
EXPT. NO. 13 13 13 13 11 11 
TEMPERATURE, .degree.C. 
100.degree. C. 
100.degree. C. 
100.degree. C. 
100.degree. C. 
110.degree. C. 
110.degree. C. 
FeSO4 PRESENT 
No Yes No Yes No Yes 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.50 0.50 0.00 0.00 0.00 0.00 
Ligand:Fe in iron oxide 
1.10 1.12 1.10 1.12 1.10 1.12 
Fe in iron oxide 
1.00 1.00 1.00 1.00 1.00 1.00 
FeSO4:Fe in iron oxide 
0 0.02 0 0.02 0 0.02 
MILLI-MOLES Ligand 19.25 19.64 19.25 19.64 19.25 
19.64 
NH3 9.63 9.82 0.00 0.00 0.00 0.00 
Fe in iron oxide 
17.50 17.50 17.50 17.50 17.50 
17.50 
FeSO4 0 0.35 0 0.35 0 0.35 
TOTAL GRAMS 15.00 15.00 15.00 15.00 15.00 
15.00 
pH initial 4.17 4.19 2.24 2.21 2.16 2.15 
final 1.93 1.64 2.46 2.36 2.31 1.84 
MINUTES TO DISSOLVE &gt;120 &lt; 150 
&gt;60 &lt; 90 
&gt;120 &lt; 150 
&gt;60 &lt; 90 
&gt;&gt;240 
&gt;90 
__________________________________________________________________________ 
&lt; 120 
LIGAND 1,3-PDTA Citric Acid 
EXPT. NO. 11 11 11 11 
TEMPERATURE, .degree.C. 
110.degree. C. 
110.degree. C. 
110.degree. C. 
110.degree. C. 
FeSO4 PRESENT 
No Yes No Yes 
__________________________________________________________________________ 
MOLE RATIO NH3:Ligand 0.50 0.50 1.00 1.00 
Ligand:Fe in iron oxide 
1.10 1.12 1.10 1.12 
Fe in iron oxide 
1.00 1.00 1.00 1.00 
FeSO4:Fe in iron oxide 
0 0.02 0 0.02 
MILLI-MOLES Ligand 19.25 19.64 19.25 19.64 
NH3 9.63 9.82 19.25 19.64 
Fe in iron oxide 
17.50 17.50 17.50 17.50 
FeSO4 0 0.35 0 0.35 
TOTAL GRAMS 15.00 15.00 15.00 15.00 
pH initial 3.17 3.20 3.36 3.37 
final 3.00 2.93 1.99 1.97 
MINUTES TO DISSOLVE &gt;150 &lt; 180 
&gt;90 &lt; 120 
&gt;240 &gt;240 
__________________________________________________________________________ 
Note: 
DTPA reacted @ 100.degree. C. within &lt;180 min, with or without FeSO4, but 
could not visually distinguish end of reaction. 
Iron compounds are expressed in terms of contained Fe 
Gamma hydrated iron oxide is Mobay Bayferrox 943