CA2214523C - Purification of metal containing solutions - Google Patents

Abstract

The present invention relates to a process for the purification of solution s containing metals comprising neutralization of the solution, reduction of Fe(III) present, and elimination of ingoing dissolved zinc, whereby the ingoing solution containi ng 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 filtratio n, 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 t he purpose intended.

Description

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 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 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 2a 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.

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.

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 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.

Claims (8)

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. 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.