PURIFICATION OF METAL CONTAINING SOLUTIONS

PURIFICATION DE SOLUTIONS CONTENANT DU METAL

English Abstract

The present invention relates to a process for the purification of solutions
containing metals comprising neutralization of the solution, reduction of
Fe(III) present, and
elimination of ingoing dissolved zinc, whereby the ingoing solution containing
metals is neutralized using magnetite and/or metallic iron, that the ingoing
iron(III) is
reduced to iron(II) by means of an addition of metallic iron, that ingoing
heavy
metals are precipitated by the addition of a precipitating amount of a
sulphide,
whereby the metal sulphides precipitated are recovered by means of filtration,
that the
solution is, optionally, subjected to an ion exchange operation to transfer
ingoing
zinc into non-complex bound form, whereupon zinc is recovered as zinc
carbonate,
that iron(II) in the remaining solution is oxidized to iron(III), whereupon
the solution
is used as such or is further treated for increasing the metal content for the
purpose
intended.

French Abstract

L'invention concerne un procédé de purification de solutions contenant des métaux, qui consiste à neutraliser la solution, à réduire le Fe(III) et à éliminer le zinc dissous entrant, la solution entrante contenant des métaux étant neutralisées à l'aide de magnétite et/ou de fer métallique, à réduire le fer (III) en fer (II) par l'addition de fer métallique, à précipiter les métaux lourds par l'addition d'une quantité précipitée de sulfure. Ainsi, les sulfures de métal précipités sont récupéreés par filtration, et la solution est éventuellement soumise à une opération d'échange d'ions de sorte que le zinc entrant soit transformé en liaison non complexe, le zinc étant récupéré sous forme de carbonate de zinc, le fer (II) dans la solution restante est oxydé et transformé en fer (III), ladite solution étant utilisée telle quelle ou encore traitée de sorte que la teneur en métaux soit augmentée selon le but recherché.

Claims

CLAIMS:
1. A process for the purification of metal-containing
solutions which contain iron salts and zinc salts,
comprising:
(a) adding magnetite and/or metallic iron to said
metal-containing solution thereby neutralizing free acid;
(b) adding a precipitating amount of sulphide to
precipitate heavy metals;
(c) filtering the solution to recover the
precipitated heavy metals; and
(d) oxidizing the iron(II) to iron(III) in the
remaining solution.
2. Process according to claim 1, wherein the process
is carried out at a temperature of 10 to 80° C.
3. Process according to claim 1 or 2, wherein
crystals of iron(II) sulphate heptahydrate are used as a
starting material of the metal containing solution.
4. Process according to claim 3, wherein the process
is carried out at 60° C.
5. Process according to any one of claims 1 to 4,
wherein iron(II) sulphate heptahydrate is added after
step (d) in order to increase the iron of the solution.
6. The process according to any one of claims 1 to 4,
further comprising after step (d);
(e) adding metals to the solution to increase
metal content.
7

7. The process according to any one of claims 1 to 6,
further comprising after step (c);
recovering zinc from the filtered solution by an
ion exchange operation with an ion exchanger and recovering
the zinc from the ion exchanger by precipitation as zinc
carbonate at about pH 8.5.
8. The process according to any one of claims 1 to 7,
further comprising after step (a);
adjusting the pH to about 4.5.
8

Description

CA 02214523 1997-09-24
WO 96!27556 PCT/SE96/00264
PURIFICATION OF METAL CONTAINING SOLUTIONS
DESCRIPTION
Technical field
The present invention relates to a process for the purification of metal
containing
solutions, in particular pickling solutions containing iron and zinc.
The object of the present invention is to obtain a process to recover metal
com-
pounds from spent solutions containing metal salts, and then in particular
zinc and
iron salts from spent pickling baths.
Background of the invention
In connection with surtace treatment of metallic objects these are pickled in
acidic
baths, whereby in particular hydrochloric acid is generally used, in order to
provide
the metal object with a clean surface to which the surface treatment adheres.
Here-
by metal and metal oxides are dissolved from the metallic objects, which metal
will
be found in the pickling solutions in the form of ions, which can be recovered
in the
form of salts of different kinds. Most of all pickling solutions containing
iron, alumi-
nium, and zinc are present, which pickling solutions ace a great problem as
they
can not be deposited in any arbitrar3r way as the metals present can create an
en-
vironmental problem. Furthermore, the metals present are a resource if they
should
be able to be recovered.
EP-A1-0 141 313 discloses a process for recovering iron and zinc from spent
pick-
ling solutions, whereby the pickling solution is first treated with an
absorption agent
to eliminate any organic material present, whereupon the iron present in three
va-
lence form is first reduced using metallic iron and zinc, to two valence form,
the zinc
present is extracted using a complex forming solvent, whereupon zinc and iron
are
recovered in a traditional way. The addition of zinc is carried out in a step
following
the addition of metallic iron in order to precipitate heavy metals present by
means
of a cementation. In connection with the cementation zinc will dissolve and
can
then be recovered as zinc chloride. When the zinc chloride has been eliminated
by
means of extraction iron can be recovered as a solution of iron(III) chloride
after oxi-
dation of the two valence iron. The process is hereby based on a cementation
of
the heavy metals present and a solvent extraction of the zinc presen by means
of
trioctyl amine in a mixture with kerosine~sodecanol. The use of the organic
solvent
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is a complication as these have to be recovered, partly for
cost reasons, partly for environmental reasons.
There is thus a desire for a more simplified and
cost efficient process for recovering the metals present,
and in particular to eliminate the need for organic
solvents.
Description of the present invention
It has now surprisingly been found possible to
meet these requirements by means of the present invention
which is characterized in that the ingoing solution
containing metals is neutralized using magnetite and/or
metallic iron, that the ingoing iron(III) is reduced to
iron(II) by means of an addition of metallic iron, that
ingoing heavy metals are precipitated by the addition of a
precipitating amount of a sulphide, whereby the metal
sulphides precipitated are recovered by means of filtration,
that the solution is, optionally, subjected to ion exchange
operation to transfer ingoing zinc into non-complex bound
form, whereupon zinc is recovered as zinc carbonate, that
iron(II) in the remaining solution is oxidized to iron(III),
whereupon the solution is used as such or is further treated
for increasing the metal content for the purpose intended.
According to one aspect of the present invention,
there is provided a process for the purification of metal-
containing solutions which contain iron salts and zinc
salts, comprising: (a) adding magnetite and/or metallic iron
to said metal-containing solution thereby neutralizing free
acid; (b) adding a precipitating amount of sulphide to
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precipitate heavy metals; (c) filtering the solution to
recover the precipitated heavy metals; and (d) oxidizing the
iron(II) to iron(III) in the remaining solution.
Further characteristics are evident from the
accompanying claims.
By means of the present invention it is obtained
that many types of spent metal containing solutions can be
recovered such as pickling baths containing hydrochloric
acid, and sulphuric acid, other liquid effluent streams,
solid disposals (materials dissolved in acid (scrap, oxides,
hydroxides, etc.)), as well as purification of iron(II)
sulphate which is a large source of iron, and which is
obtained as a residual product from other processes, such as
the production of titanium; free acid in the ingoing baths
can easily be neutralized using different sources of iron,
whereby, by using different oxides, the hydrogen production
is eliminated; optional unbalanced charges of the ingoing
solution can be adjusted in a simple way before treatment;
the source of sulphide can be varied in a simple way
depending upon the local conditions; Zn ion exchange can be
carried out either prior to or after the precipitation of
the sulphides, which increases the flexibility of the
process, whereby it should be noted that when chloride based
solutions are not present, Zn can be precipitated as
sulphide; the waste material generated by the process is
present as
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CA 02214523 1997-09-24
WO 96/27556 PCT/SE96/00264
a hydroxide-sulphide sludge, which is more easily taken care of than pure
hydroxi-
de sludges; Zn which is recovered as a zinc carbonate is a valuable product.
The process of the present invention is carried out in accordance with the
folowing:
An ingoing pickling bath containing iron and zinc, whereby the iron can be
present
in partly two valence form, partly three valence form, is neutralized
depending on its
zinc content in two different ways, viz., if the zinc content is low, first by
using mag-
netite, whereby the acid present is neutralized, and one obtains an iron(III)
chloride
or sulphate depending on the composition of the ingoing pickling bath,
whereupon
the iron(III) content of the solution is reduced by adding metallic iron,
which reduc-
tion is of importance for the subsequent working-up of the solution. If the
zinc con-
tent is high a neutralization is carried out directly using metallic iron.
Then, inde-
pendent of the zinc content, there is a further neutralization using hydroxide
or car-
bonate, to about pH 4.5 in order to achieve optimal conditions for the
subsequent
sulphide precipitation, which is carried out using sodium sulphide, sodium
hydro-
gen sulphide or another sulphide source such as given above. At the addition
of
sulphide the ingoing heavy metals are precipitated, and then in particular Cu,
Ni,
Pb, Cr, and Co. The sulphide precipitation is optimal at 50-60°C. Then
a filtration is
carried out to eliminate sulphides precipitated, whereby the ingoing
hydroxides are
eliminated as well. The solution, now being substantially freed from said
heavy me-
tals is allowed to pass a Zn ion exchanger where the ingoing Zn ions in
complex
form present in the solution are eliminated and are obtained in a raffinate
stream
which is treated per se in accordance v~,~ith belo4v. The solution now
substantially
containing iron(II) ions is subject to an oxidation by means of chlorine or
oxygen. At
the oxidation using oxygen pH is suitably adjusted by means of an addition of
sul-
phuric acid. Oxidation using chlorine is normally carried out when one has a
chlo-
ride containing solution. The iron(III) containing solution thus obtained has
usually
too low an iron(III) content to be used as sewage water treatment agent as end
use, and thus further a iron(!I!) compound is added to reach a suitable
concentra-
tion for the user, viz. about 12% Fe(III). The addition of further iron can be
done by
adding "copperas", i.e. iron(II) sulphate heptahydrate, as well, which is
dissolved
and participates in the oxidation process.
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Ingoing chemical reactions are the following:
1. Removal of free acid (neutralization):
A) Magnetite: Fe203 + 6HCI = 2FeCl3 + 3H20
Fe0 + 2HCI = FeCl2 + H20
B) Fe-metal: Fe + 2HC1 = FeCl2 + H2
2. Ferri-reduction
Fe-metal 2Fe3+ + Fe = 3Fe2+
3. Sulphide precipitation
A) Neutralization: Cr3+ + 30H- = Cr(OH)3
B) Metal sulphides: Pb2+ + S2- = PbS
4. Oxidation
A) using chlorine: 2FeCl2 + Cl2 = 2FeCl3
B) using oxygen: 2FeS04 +0,502 + H2S04 = Fe2(S04)3 + H20
The zinc containing raffinate from the ion exchanger is precipitated in two
separate
steps using an intermediate filtration. Primarily, sodium carbonate is added
to raise
pH to about 5, whereby ferri hydroxide is precipitated and is separated (in
the case
the zinc ion exchanger preceeds the sulphide precipitation), whereafter
further so-
diem carbonate is added until zinc carbonate is precipitated which occurs at
about
pH 8.5. Zinc carbonate is filtered off, whereby a zinc carbonate is obtained
having
the following typical composition: >95% Zn-carbonate, <5% Fe-carbonate.
Further,
there are trace elements present. Cr 15 mg/kg; Cu 5 mg/kg; Mn 140 mg/kg; Co <1
mg/kg; Ni 15 mg/kg; Pb 50 mg/kg; och Cd <1 mg/kg.
Iron{II) sulphate heptahydrate can be purified in accordance with the present
inven-
tion whereby the iron(!I) sulphate heptahydrate crystals are first dissolved
in an aq-
uoeus solution at 60°C, where the dissolution maximum is at hand.
Hereby, a
iron(I I) sulphate solution having 10 to 11 % of iron. The process is run in
accor-
dance with the above at 60°C all the time in order to maximize the
concentration.
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However, the product solution need not be run through a Zn ion exchanger as no
chloride system is present, but only a sulphate system. When the sulphide
precipi-
tation has been carried out the temperature is lowered to 15 to 20°C
i.e ambient
temperature, pure iron(II) sulphate heptahydrate crystals being precipitated.
The present process does not require any substantial addition of energy, but
the
? ingoing reactions can be carried out at 10 to 80°C abd thereby in
general at ambi-
ent temperature, with the exception of the purification of iron(II) sulphate
hepta
hydrate in accordance with the above, where one runs the process at
60°C for
reasons of optimization.
Below the results of a series of runs using the present process follow:
Example 1 Pickling bath (NCI) having a tow concentration of Zn
Reduction of acid magnetite 30g/1000g of pickling bath
Ferri reduction iron powder 10g/1000g of pickling bath
Neutralization NaOH -->pH 4,5
S2- precipitation Na2S (60% Na2S) 4g/1000g of pickling bath
Zn ion exchange Amberiite IRA 420
Fe HCICu Ni Pb Cr Co Zn Mn
g/1 g/1ppm ppm ppm ppm ppm ppm ppm
Pickling bath in 111 50 5 31 1170 325 4 128 788
After neutr. 149 0 3 32 1162 2 5 130 862
After S2- precipita.151 0 <1 <1 4 <1 <1 130 910
After Zn ion exch.150 0 <1 <1 2 <1 <1 2 900
Example 2 Pickling bath (HCI) having a high concentration of Zn
Reduction of acid iron powder
Ferri-reduction iron powder
Zn ion exchange Purolite A500
Neutralization NaOH -->pH 4,5
S2- precipitation Sulphide sol. (5% Na2S, 8% NaHS, 10% Na2C03
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Fe HCI Cu Ni Pb Cr Co Zn Mn
gn g~ ppm ppm ppm ppm ppm ppm ppm
Pickling bath in 135 30 - 42 200 59 4 3012 740
After neutr. 190 0 - 66 206 78 5 2992 740
~ After S2- precipita.190 0 - 66 206 78 5 <10 740
After Zn ion exch. 150 0 - <1 <2 <1 <1 <10 740
The solution purified can suitably be used for the purification of sewage
water
where the contents of ferri chloride is a flocculating agent for suspended and
col-
loidal materials present in the sewage water.
20
30
6