TECHNICAL FIELD The present invention relates to a method of obtaining an essentially pure iron salt from acidic, metal contaminated water comprising Fe2+, at the same time as the water is purified. The method has been developed mainly for purification of acidic, metal contaminated mine waters, but may also be utilised on other types of acidic, metal contaminated waters comprising Fe2+.

PRIOR ART AND PROBLEMS Acidic water residuals originating from mining or similar activities, almost always contain a mixture of different metals, which in large scale may be difficult to separate from each other in an economically feasible way. The actual purification of the water is usually no problem as such, but the problem lies in that in conventional purification the metals are obtained mixed with each other or at a purity which is too low for them to be utilised for other purposes. Instead, the metals separated from the water must be dum- ped, resulting only in costs and giving rise to a risk of negative environmental effects.

Known patents relating to the treatment of metal contaminated waters are e. g. JP 1315389, JP 1090089, JP 61278397, JP 61146389, JP 59186694, JP 59154192, JP 58131190, JP 61204088, JP 58205580 and US 5043078.

BRIEF ACCOUNT OF THE INVENTION The present invention aims at obtaining an essentially pure iron salt from an acidic, metal contaminated water comprising Fe2+, which iron salt preferably is in such a state and concentration and is so pure that it may be sold as a product as such, preferably as a chemical for water purification, whereby it may cover a major or minor part of the operating costs for the purification of the water. Another purpose is to also obtain other metals/metal salts from the contaminated water in such a way that also these are obtained in such a state and concentration and at so high a purity that they may be sold as products as such. Preferred examples of such metal salts are copper-, zinc-and or aluminium salts. Moreover, it is a primary purpose of the invention that the water obtained after the last step of the purification should be pure enough to be used as process water or to be released into the recipient. Most preferred, there is no rest products in appreciable amounts after the treatment according to the invention, which rest products would have to be dumped. Instead, the different metals have been

separately recovered from the contaminated water, in their main contents, as different metal salts.

These and other purposes are achieved by the method according to the invention as defined by patent claim 1.

A fundamental idea in the method of treatment according to the invention, is that a cationactive ion-exchange compound which is selective for trivalent iron, Fe3+, is used for the recovery of essentially pure iron salt. Accordingly, this ion-exchange compound is of a certain, carefully chosen, type which in the following is denoted"type A". An ion-exchange compound of type A is a strongly acidic ion-exchange compound, which is mainly selective for copper but also for trivalent iron. Moreover, it has the capability to catch certain other metals present at low concentrations in the contaminated water, however not Fe2+. An ion-exchange compound of type A typically consists of an acidic ion-exchange compound or an acidic ion-exchange resin, having a Fe3+/Zn2+ selectivity larger than 1000 at a pH < 2.5, thus leading to a 1000 times more Fe3+ than Zn2+ ad- sorbing on the compound, based on weight. Such a ion-exchange compound may be constituted by Amberlite IRC 748 (sold by Rohm and Haas) or Lewatit TP 207 (sold by Bayer), for example.

As a consequence to the fact that the ion-exchange compound A also catches certain other contaminating metals in addition to just Fe3+, a difficulty arises in obtaining an essentially pure salt of trivalent iron by aid of this ion-exchange compound A.

According to the invention, this problem is solved by the treatment being performed in several steps, the same type of ion-exchange compound, type A, being used in at least two steps. In the first step, hereafter denoted step a, ions of copper and trivalent iron present in the water are caught in a first ion-exchange column, but also certain other metal ions present in the water are caught in this ion-exchange column, which means that the water is purified also in respect of these other metal ions. The amount of trivalent iron caught in this first step may be controlled thanks to the ion-exchange compound of type A mainly being selective to copper, which means that trivalent iron already caught can be replaced by copper if the column is operated to full capacity.

Thereafter, in a step b, Fe2+ ions present in the water are in essential completely oxidised to Fe3+ ions. These newly formed Fe ions are thereafter recovered in a following step, step c, in a second ion-exchange column having an ion-exchange compound of the same type, type A, as the ion-exchange compound used in step a.

Thanks to the other metals that also can be caught by the ion-exchange compound of type A, already having been removed from the water in step a, essentially only Fe ions will be recovered from the water in step c. Thereafter, the ion-exchange compound A from step c may be regenerated in a subsequent step, step d, by aid of an acid, e. g. hydrochloric acid or sulphuric acid, a solution of essentially pure iron (III) salt, e. g. iron (III) sulphate or iron (III) chloride, being obtained. Suitably, the purification of the water thereafter continues in one or more additional steps, adapted to remove other contaminants from the water, usually metal contaminants. An alternative is of course that such other contaminants are removed from the water already before the recovery of Fe3+ ions or possibly between the at least two steps that comprise treatment with ion- exchange compound of type A.

DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in greater detail with reference to Fig. 1 which shows a block diagram of a treatment process according to a preferred embodiment of the invention.

In the process according to Fig. 1, an incoming, metal contaminated water, e. g. a mine water, which typically has a pH of 1-5, is treated. The method according to the invention is applicable on water having contaminant concentrations in a large span of concentrations according to the below.

Sulphate 0.1-80 g/1 Total Fe 0.1-20 g/l Fe2+ 0. 1-20 g/l<BR> Fe3+ 0-20 g/1 Zn 0.1-7 g/1 Mg 0-10 g/1 Na 0-10 g/1 Ca 0-1 g/1 Al 0-5 g/1 Mn 0-5 g/1 Cu 0-1 g/1 Pb 0-1 g/1 Co, Cd, Sr, Ni, Cr, Li, As, Mo and others < 0.1 g/1 each.

Optional introductory step, step g In an optional, but not always necessary step, here denoted step g, a part of the bivalent iron (Fe2+) in the water is oxidised to the trivalent state (Fe3+) by aid of air 24. The oxidation of Fe2+ to Fe by air is what normally occurs in nature. However, at the low pH of the water/mine water this oxidation proceeds very slowly. Consequently, this oxidation step is preferably performed at pressurised conditions, which will give a considerably shortened reaction time compared to an oxidation step with air which is performed at atmospheric conditions. E. g. at a temperature of 15-25'C and a pressure of about 10 bar, about 20 % of the bivalent iron present in the water may be oxidised to ~trivalent iron in about 10-15 minutes. Preferred pressurised conditions may be realised by e. g. finely distributed air being supplied by aid of a compressor, to the water after the pump down in the mine, i. e. where an over-pressure exists due to the water head, or by the air being supplied to a pressurised reactor above ground. Possibly, a minor adjust- ment of water pH may be required, more precisely a lowering of pH to about 1.6-2.0, preferably about 1.8-1.9, before, during or directly after the oxidation step g, in order to avoid precipitation of iron hydroxide or jarosite. The purpose of the oxidation step g is to make use of the cheapest possible oxidisation agent, i. e. air, in order to perform a part of the oxidation of Fez+ to Fe3+, required for the invention. The final goal is a complete oxidation, which is performed in a subsequent oxidation step, step b.

Step a In a treatment step, step a, directly following step g, copper, an optionally desired amount of trivalent iron and certain other metal contaminants possibly present, such as vanadium and nickel, are recovered by aid of the cation-exchange compound of type A, described above. The ion-exchange device is suitably of conventional type having several columns, e. g. three columns where two are alternatingly connected in series while a third is regenerated. It is however also possible to use a continuous ion- exchanger if requirements exists on a lower residual concentration in the water and a higher concentration in the recovered concentrate 3. The ion-exchanger is regenerated by acid 2, preferably hydrochloric acid or sulphuric acid. The recovered concentrate 3 comprises copper salt as copper sulphate or copper chloride, and possibly trivalent iron as e. g. iron (III) sulphate or iron (III) chloride, which after increase of the concentration, e. g. by evaporation 22, forms a product 23 which may be used in connection with the manufacturing of red colouring pigment, e. g. In certain mine waters, the concentration of copper is however low, in which case the purpose of step a mainly is to remove other possible metals that get caught on ion-exchange compound of type A, so that these will not contaminate the iron salt which is recovered in subsequent step (s) (step c-d). On the

other hand, if the recovery of essentially pure copper is of special interest, step a is adapted to recover as little trivalent iron as possible in this step. In this case it may also be preferred to execute step g after step a instead of before step a.

Step b In step b, a complete or essentially complete oxidation of Fe2+ to Fe is performed.

Most preferred, this oxidation is performed in two or more steps, the water being treated with different oxidisation agents of successively increasing oxidising effect. First, an oxidation with air is performed, possibly at pressurised conditions in the same or similar way as in step g. Thereafter, oxidation with oxygen gas is performed and finally with hydrogen peroxide or possibly ozone. Also the oxidation with oxygen gas, when such is used, is preferably performed at pressurised conditions. An alternative is that the oxidation is performed without utilisation of air or oxygen gas, economical or practical aspects mainly deciding which oxidisation agent or agents should be used. Addition of air and/or oxygen gas is symbolically illustrated by arrow 4 in the figure, while addition of stronger oxidising agents is illustrated by arrow 5. The pH is adjusted by an acid, before, during or after oxidation, so that a final pH below 2 is obtained after the oxidation.

Step c-d In step c, the trivalent iron is recovered by aid of an ion-exchange compound of the same type, type A, as the one used in step a. By the concept of"of the same type"it should here be understood that it is most preferred that exactly the same type of ion- exchange compound is used both in step a and in step c, but that it also is conceivable that two different sorts of ion-exchange compounds are used, both of them however falling under the definition of"type A"which has been done above.

The incoming water to the ion-exchange step c should have a pH of between 1 and 2.

The best result is obtainable when a continuously operating ion-exchange column is used, e. g. of the type called ISEP, it however also being possible to use conventional ion-exchange equipment, i. e. an equipment comprising two or more columns which are successively regenerated during the course of the process. In step d, the ion-exchange compound is regenerated by an acid 6, preferably hydrochloric acid or sulphuric acid, the concentrate thereby obtained being a solution of a Fe3+ salt, e. g. iron (III) chloride or iron (III) sulphate. Thanks to the ion-exchange compound being selective to Fe3+, and thanks to the fact that such an ion-exchange compound already has been used in a previous step, whereby other metals which also get caught on this type of ion-exchange

compound already have been removed, a very pure iron product may be achieved in this step. Preferably, the purity is so high that the recovered iron product fulfils the require- ments to be used as a water purification chemical. However, the product 7, which is obtained, has a concentration which is too low for it to be able to be sold as a merchan- dise, which means that it has to be concentrated by e. g. crystallisation or evaporation 8.

After evaporation, the product 9 has a concentration of at least 7 weight-%, preferably about 10-13 weight-%, calculated on Fe. After crystallisation, the product 9 consists of a ferric salt, such as iron (III) sulphate. Any free acid remaining after the crystallisation step may be recirculated to an optional step of the process, where acid is required.

Optionally, product from step c may be mixed into product from step a for use in the manufacturing of red paint. The water which exits from the ion-exchange equipment is preferably led to the next step.

Step e In step e, a continuing purification of the water takes place in respect of other metals or other contaminants, here exemplified by zinc. The recovery of zinc may be performed either by ion-exchanging or by precipitation, preferably as zinc sulphide.

In the case of ion-exchanging, use is made of an ion-exchange compound, type B, which is selective to zinc. The ion-exchange compound of type B is preferably an acidic or neutral ion-exchange compound or an ion-exchange resin, having a selectivity Zn2+/Ca2+ larger than 100, and selectivity Zn/Al larger than 100, at pH < 7 and calculated on weight. Before the ion-exchanging, the pH of the water is increased by alkali 10, e. g. magnesium oxide or hydroxide or sodium hydroxide, to a suitable level depending on which type of ion-exchange compound that is used. Regeneration of the ion-exchange compound is performed by aid of an acid 11, e. g. hydrochloric acid or sulphuric acid, whereafter the concentrate 12 obtained, which comprises a zinc salt, e. g. zinc chloride or zinc sulphate, is further concentrated, e. g. by evaporation 13. The product 14 obtained after evaporation is of adequate concentration and purity in order for it to be sold as a merchandise.

When on the other hand precipitation of zinc is utilised, this is performed after a possible adjusting of pH with alkali 10, suitably magnesium oxide or hydroxide or sodium hydroxide, to bring pH to 2-5 after the precipitation, preferably 3-4 and even more preferred about 3.5. Preferably, zinc is precipitated as zinc sulphide, in which case use may be made of e. g. sodium sulphide or green liquor from a sulphate pulp mill, as a source of sulphide 15. The formed precipitate (ZnS) may optionally be flocculated by

addition of a flocculation agent 16, e. g. poly-acrylamide. As an alternative to the addition of a flocculation agent, or in combination, the sludge may recirculated to the alkali addition step or just before the addition of a precipitation chemical 15, in order to the improve flocculation. The sludge 17 is thereafter separated by e. g. sedimentation, flotation or filtering, whereafter the sludge may be dewatered in a press, optionally in connection with washing of the sludge if a requirement to remove solute salts from the sludge exists. The zinc product 14 obtained, exhibits adequate concentration and purity to be sold as a merchandise. One advantage is attained in that a more pure zinc product may be achieved when the iron already has been removed, in comparison with the case if zinc was to be recovered before removal of the iron.

Step f By addition of alkali 18, suitably magnesium hydroxide or sodium hydroxide, to the water from step e, there is achieved an increase of the pH of the water, to a pH of about 5-9, preferably 6-7 and even more preferred 6-6.5. This results in a precipitation of the aluminium present in the water. The formed precipitate 19 may optionally be flocculated by addition of a flocculation agent, e. g. poly-acrylamide, before being separated 20 by e. g. sedimentation, flotation or filtering, whereafter the sludge may be dewatered in a press, optionally in connection with washing of the sludge if a requirement to remove solute salts from the sludge exists. The aluminium product obtained, exhibits adequate concentration and purity to be sold as a merchandise, e. g. for the manufacturing of chemicals for purification of water. One possible use may be to mix it into the concentrated ferric salt solution from the ion-exchange step c-d.

The water 21 obtained after step f will be of such a quality and purity that it in most cases may be used as process water or may be released to a recipient without any further purification activities. Hereby, it should be understood that by the concept of pure water, which may be released to a recipient, it is meant water that does not contain compounds in amounts harmful to the nature, whereas however certain magnesium salts and/or calcium salts e. g., may remain in the water.

EXAMPLE.

An experiment was performed, in which an incoming water of the composition shown in table 1, was purified. No introductory oxidation was performed, i. e. no step g. The ion-exchange compound Amberlite IRC 748 (Rohm and Haas) was used in step a, as well as in step c. The oxidation in step b, between steps a and c, was performed by use of peroxide only. Zinc was precipitated in step e as zinc sulphide at pH 3.5, by addition of green liquor. In table 1, there is shown the composition of the water after the different steps, as well as the composition of the iron product recovered in step c.

Table 1 Incoming After step a After step c After step e After step f Fe product water from step c Fe, 6000 5400 5 5 0.2 23 000 mu/1 Cu, 15 0.1 0.02 0.01 0.01 0.3 mu/1 Zn, 1500 1500 1500 2 0. 5 2 mg/1 Al, 300 300 300 300 0.5 5 mu/1 Ca, 400 400 400 400 400 20 mu/1

The invention is not limited to the embodiments described above, but may be varied within the scope of the claims. Thus, it should be realised e. g. that although it is preferred according to the invention that only the steps described above are performed directly after each other, it is however also conceivable that also other steps, known per se and adapted and optimised to remove other contaminants, usually metal contami- nants, may be used in the purification process in any optional position.