Process for the selective removal of ferric ion from an aqueous solution containing ferric and other metal ions

A process for treating an aqueous acidic solution containing a large amount of ferric ion and other metal ions such as a hydrochloric acid solution of manganese nodules, which includes contacting the aqueous solution with an organic extractant to selectively extract the ferric ion therefrom. The extractant is comprised of a dialkylphosphoric acid having 6-12 carbon atoms and a hydrocarbon solvent such as kerosene. The aqueous phase obtained after the extraction is then added with a pH-controlling agent to increase the pH thereof to a value so that unextracted ferric ion remaining therein may precipitate as ferric hydroxide while the other metal ions may be prevented from precipitating. Solid-liquid separation of the resulting mixture gives a solution having a ferric ion concentration of about 0.1 g/l or less. The pH-controlling agent is preferably an industrial waste containing a metal oxide or hydroxide such as a sludge from plating industries. By repeating the above procedure, a substantially ferric ion-free solution may be obtained.

BACKGROUND OF THE INVENTION 
This invention relates to a process for treating an aqueous solution 
containing ferric and other metal ions for selectively removing the ferric 
ion. 
When ferric ion is intended to be selectively separated from an acidic 
solution containing various metal ions, a neutralising agent such as 
calcium carbonate or calcium hydroxide is generally added to the acidic 
solution to selectively precipitate the ferric ion as ferric hydroxide. 
This method is, however, not applicable to an acidic solution having a 
high ferric ion concentration such as a hydrochloric acid solution of 
manganese nodules, because colloidal ferric hydroxide is produced in such 
a large amount that a solid-liquid separation is inhibited. Moreover, this 
method when applied to a large scale treatment, is disadvantageous from 
economic point of view because the neutralising agent is not available at 
a low cost. 
Recent trend of metal refining industries is towards hydro-metallurgy in 
favor of its decreased energy consumption. Thus, there is a great demand 
to develop a simple and economical process for removing ferric ion from 
acidic aqueous solutions containing various metal ions, which removal is 
indispensable to selectively recover desired metals therefrom. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a process 
which can selectively and completely separate ferric ion from a solution 
containing a large quantity of ferric ion and various other metal ions. 
Another object of the present invention is to provide a simple and 
economical process for treating a ferric ion-containing aqueous solution, 
especially a hydrochloric acid solution of manganese nodules, for 
selectively removing the ferric ion therefrom. 
A further object of the present invention is to provide a process which 
permits the use of industrial wastes, such as sludges from plating 
industries, for the removal of ferric ion from an aqueous solution of the 
above-mentioned type. 
In accomplishing the foregoing objects, there is provided in accordance 
with the present invention a process for treating a ferric ion-containing 
aqueous solution for the removal of ferric ion, which includes the steps 
of: 
(a) contacting the ferric ion-containing aqueous solution with an organic 
extractant including a dialkylphosphoric acid having 6 to 12 carbon atoms 
and a hydrocarbon solvent to extract the ferric ion therefrom, thereby 
obtaining an organic phase containing the extracted ferric component and 
an aqueous phase containing residual unextracted ferric ion; 
(b) separating the aqueous phase from the organic phase; 
(c) mixing the aqueous phase with a pH-controlling agent to increase the pH 
thereof to a value sufficient to precipitate at least a part of the 
residual ferric ion as ferric hydroxide; and 
(d) separating the mixture from step (c) into a solid phase and a liquid 
phase. 
Preferably, the pH-controlling agent is an ore or an industrial waste 
containing metal oxides or hydroxides, such as a sludge from plating 
industries. 
Other objects, features and advantages of the present invention will become 
apparent from the detailed description of the invention to follow. 
DETAILED DESCRIPTION OF THE INVENTION 
Ferric ion-containing aqueous solution to be treated in accordance with the 
process of the present invention has, generally, a ferric ion content of 
5-100 g/l and a pH value of 0-2 and contains one or more metal ions other 
than ferric ion. Examples of such aqueous solutions include a hydrochloric 
acid solution of manganese nodules (ferric ion concentration: about 10-50 
g/l) and a solution obtained by treating manganese nodules for leaching 
with a waste pickle liquor (ferric ion concentration: about 10-50 g/l). 
In the first step of the process of this invention, the aqueous acidic 
solution is contacted with an organic extractant to selectively extract as 
much ferric ion as possible therefrom (step (a)). The extractant comprises 
a hydrocarbon solvent and a dialkylphosphoric acid having 6-12 carbon 
atoms. 
The content of the dialkylphosphoric acid in the extractant is 5-50% by 
volume. Di-2-ethylhexylphosphoric acid is easily commercially available 
and, therefore, the use thereof as the extractant is preferable. The 
amount of the extractant used depends on the concentration of the ferric 
ion in the aqueous solution to be treated and the concentration of the 
dialkylphosphoric acid in the extractant. Generally, the extractant is 
used in an amount of 0.1 to 10 parts by volume per one part by volume of 
the aqueous solution to be treated. 
Extraction of ferric ion from the aqueous solution is considered to proceed 
according to the following chemical equation: 
EQU Fe.sup.3+ +3R.H.revreaction.R.sub.3.Fe+3H.sup.+ (I) 
where R.H stands for a dialkylphosphoric acid. Thus, the organic phase 
after step (a) contains the extracted ferric component. On the other hand, 
the aqueous phase contains a small amount of residual, unextracted ferric 
ion and substantially all of the other metal ions and has a higher 
activity than the aqueous solution prior to step (a). 
In the next step, the organic phase is separated from the aqueous phase 
(step (b)). If desired, the extraction and separation steps (steps (a) and 
(b)) may be repeated twice or more times. It is preferred that the 
extraction be conducted until the concentration of ferric ion in the 
aqueous phase becomes about 1 g/l or less. 
The ferric component-containing organic phase obtained in step (b) may be 
treated with 6-8 N hydrochloric acid to extract the ferric component. The 
thus regenerated organic phase may be recycled to step (a) for use as the 
extractant. 
The aqueous phase from step (b) is then added with a pH-controlling agent 
to increase the pH thereof to a value sufficient to precipitate at least a 
part of the residual unextracted ferric ion remaining in the aqueous phase 
as ferric hydroxide (step (c)). 
Although the pH control can be achieved with the use of sodium hydroxide, 
calcium carbonate or other alkaline materials, it is preferable to use an 
industrial waste containing a metal oxide or oxides capable of being 
dissolved in acidic aqueous solution and of increasing the pH thereof. 
Illustrative of the wastes are sludges from various plating industries 
containing one or more metal hydroxides such as copper hydroxide, nickel 
hydroxide and zinc hydroxide, and dusts from metal refining steps 
containing one or more metal oxides such as copper oxide, lead oxide and 
zinc oxide. Further, ores such as copper oxide ores, manganese oxide ores 
and zinc oxide ores may be advantageously used as the pH-controlling 
agent. The wastes and ores may serve to function not only as 
pH-controlling agent but also as a part of the source or raw material of 
metals to be recovered. Therefore, the use of them is very advantageous 
because they are available at low economic prices and because the yield of 
the metals will be increased as much. 
In step (c), the pH-controlling agent is added in an amount so that the 
residual ferric ion remaining in the aqueous phase may be precipitated as 
much as possible while preventing the precipitation of the other metal 
ions. Generally, the pH of the aqueous phase is controlled in step (c) to 
about 1.5-3. 
Preferably, the mixture obtained by the addition of the pH-controlling 
agent to the aqueous phase is maintained at a temperature of 
50.degree.-100.degree. C., preferably 80.degree.-95.degree. C., under 
stirring for 30-60 min. so as to accelerate the reactions and to make the 
subsequent solid-liquid separation easy. 
The resulting mixture obtained in step (c) is then separated into a solid 
phase and a liquid phase by, preferably, filtration (step (d)). Since the 
amount of the residual ferric ion in the aqueous phase subjected to the 
previous pH-controlling step (c) is small, the amount of the ferric 
hydroxide is small. Therefore, the solid-liquid separation may be 
performed without difficulties. 
Meanwhile, the pH-controlling agent used in step (c) may contain matters 
insoluble in acidic aqueous solution. Such insoluble matters can be 
separated in step (d) together with the ferric hydroxide. Thus, the 
process of this invention permits the use of a variety of industrial 
wastes as long as they contain acid-consuming components and do not 
produce substances which inhibit solid-liquid separation. 
The liquid phase resulting from step (d) has a ferric ion concentration of 
0.1 g/l or less. If necessary, the liquid phase may be subjected to the 
treatment of steps (a) through (d) again. By this repetition, it is 
possible to obtain a substantially ferric ion-free liquid phase. The 
liquid phase is then subjected to a further treatment for the selective 
recovery of respective metals. This can be done by any known ways. 
However, this is beyond the scope of the present invention and no 
description is herein made.

The following example will further illustrate the present invention. 
EXAMPLE 
An acidic aqueous solution obtained by dissolving manganese nodules in 
conc. hydrochloric acid was subjected to the treatment according to the 
present invention. The aqueous solution contained 35 g/l of ferric ion, 30 
g/l of ferrous ion, 0.5 g/l of cupric ion, 0.7 g/l of nickel ion, 0.1 g/l 
of cobalt ion and 18 g/l of manganese ion and had a pH value of 0.1. 
The aqueous solution was first added with an organic extractant containing 
40% by volume of di-2-ethylhexylphosphoric acid and 60% by volume of 
kerosene, in a by volume ratio of the extractant to the aqueous solution 
of 5:1. After vigorous shaking for extraction, the aqueous phase was 
separated from the organic phase. The aqueous phase was found to contain 5 
g/l of ferric ion and 20 g/l of ferrous ion and to have a pH of below 0. 
The above solvent extraction was repeated once more for the thus obtained 
aqueous phase with an extractant to aqueous phase ratio of 5:2, whereby to 
obtain a second aqueous phase having a ferric ion concentration of 1 g/l, 
a ferrous ion concentration of 20 g/l and a pH value of below 0. An 
analysis of the second aqueous phase revealed that there was no noticeable 
change in concentrations of copper, nickel, cobalt and manganese 
throughout the extraction. 
The second aqueous phase was then added with a plating sludge containing 
1.00 wt % of Na, 2.50 wt % of Mg, 6.81 wt % of Ca, 12.00 wt % of Cr, 0.70 
wt % of Fe, 6.10 wt % of Ni, 11.10 wt % of Cu, 2.70 wt % of Zn, 0.29 wt % 
of Al and 4.10 wt % of SiO.sub.2, in an amount of 0.25 Kg per one liter of 
the second aqueous phase. The mixture was then heated to 90.degree. C. and 
maintained at that temperature for 30 min. with stirring. Thereafter, the 
solid phase was filtered off to obtain a third liquid phase having a pH 
value of 1.5, a ferric ion concentration of 0.1 g/l and a ferrous ion 
concentration of 15 g/l. The filtration was able to be effected with ease. 
The third aqueous phase was further subjected to a solvent extraction in 
the same manner as above with an extractant to aqueous phase ratio of 5:2 
(vol/vol) to obtain a fourth aqueous phase having a pH value of 0.8, a 
ferric ion concentration of 0.1 g/l and a ferrous ion concentration of 13 
g/l. 
The fourth aqueous phase was added with the sludge in an amount of 0.1 Kg 
per liter of the fourth aqueous phase and the mixture was maintained at 
90.degree. C. for 30 min. under stirring. A solid-liquid separation of the 
mixture gave a fifth aqueous phase having a pH value of 2.5 and a ferrous 
ion concentration of 10 g/l. No ferric ion was detected. 
The ferric component-containing organic extractant obtained after the above 
extraction procedures was treated with 6-8 N of hydrochloric acid to 
remove the ferric component by extraction. The thus regenerated extraction 
was able to be reused for extraction purpose. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.