NO743908L - - Google Patents

Description

Process for removing zinc from an aqueous solution.

The present invention relates to a method for removing zinc from an aqueous solution, in particular zinc-containing aqueous solutions of the type obtained as waste water or a diluted process solution in the manufacture of rayon. One purpose of the invention is to remove zinc from an aqueous zinc solution. so that this, with regard to remaining zinc, can be used again in the process or discharged to a recipient. A further purpose of the invention is to remove zinc from the aqueous solution when producing rayon, so that the solution can again be used in the process.

When producing rayon according to the viscose process, a so-called viscose is first produced. This is made from cellulose which is treated with alkali to give alkali cellulose, which is then matured and treated with carbon disulphide and then dissolved in a dilute sodium hydroxide solution. After further maturation, filtration, removal of dissolved gases etc, the viscose is ready for spinning. This is done by injecting the viscose, consisting essentially of cellulose xanthogenate, through openings and into a spinning bath, which is an aqueous solution containing essentially sodium sulphate, sulfuric acid and zinc sulphate. When the cellulose xanthogenate comes into contact with the spinning bath, cellulose is regenerated. During the spinning process, the cellulose usually forms threads or ribbons which are then subjected to a stretching process with the aim of orienting the cellulose molecules in the longitudinal direction of the thread or ribbon, thereby achieving the desired strength. During the stretching process, the cellulose must not be completely regenerated. In order to regulate the regeneration rate, for example, a certain zinc concentration is kept in the spinning bath. The zinc ions beyond a delaying effect on the regeneration. Until now, no economical method has been found for the recovery of zinc from very dilute spinning baths or zinc-containing wash water.

The zinc-containing water from rayon production can have a varying composition. In order to enable the treatment by the method according to the present invention, it is recommended that the zinc-containing solution contains an amount of free sulfuric acid that is less than 3o g/l, a total content of sulfate ions of loo - loo g/l and a zinc content of the range o,ool - lo g/l. The content of sodium ions in the aqueous solution is of no importance. If the aqueous solution has a different composition, it is recommended that the composition be adjusted to what is shown above, for example by dilution or neutralisation.

The method according to the present invention is characterized by the fact that the aqueous solution is treated in an extraction step with an organic solution containing an alkylphosphoric acid, whereby zinc and the alkylphosphoric acid form a chemical compound,

which is more soluble in the organic solution than in the aqueous solution

solution and is thus extracted from the aqueous solution, after which the organic solution containing zinc is treated in a stripping step with an aqueous solution with such a composition that the chemical compound is split and zinc is transferred to the aqueous solution, after which the zinc-free organic solution is returned for renewed extraction, either directly or after certain parts of it have been freed from impurities.

The alkyl phosphoric acid can, for example, be a monoalkyl phosphoric acid

with the general formula

or a dialkyl phosphoric acid of the general formula

The organic group R (i-dialkyl phosphoric acid, the groups may be the same or different) comprises a branched or unbranched hydrocarbon group, suitably containing at least 5 carbon atoms, so that the alkyl phosphoric acid becomes negligibly soluble in water.

For reasons of solubility, substituents with a hydrophilic nature reside

is avoided in the R group's hydrocarbon chain. In the present description, the acidic organophosphorus compounds (1) - (5) are generally designated as alkyl or dialkyl phosphoric acid. More precisely, the compounds (1) should be referred to as monoalkylphosphoric acids, the compounds (2) as monoalkylphosphonic acids, the compounds (3) as dialkylphosphoric acids, the compounds (4) as alkylalkylphosphonic acids and the compounds (5) as dialkylphosphinic acids. Useful compounds of type (3) are di-(2-ethylhexoxy)-phosphoric acid, di-(3, 5, 5-trimethylhexoxy)-phosphoric acid,

di-(cyclohexoxy)phosphoric acid,

di-(phenoxy)-phosphoric acid, phenoxy-(2-ethylhexoxy)phosphoric acid,

phenoxy-propoxy-phosphoric acid and

phenoxy-cyclohexoxy-phosphoric acid. r

"Useful" compounds of the type (4) are

(2-ethylhexoxy)-(2-ethylhexyl)-phosphonic acid,

(2-ethylhexoxy)-phenyl-phosphonic acid,

(n-butoxy)-phenyl-phosphonic acid,

phenoxy-cyclohexyl-phosphonic acid,

phenoxy-phenyl-phosphonic acid,

cyclohexoxy-cyclohexyl-phosphonic acid, and

methyl isobutoxy-(n-octyl)-phosphonic acid.

Useful compounds of type (5) are di-(2-ethylhexyl)-phosphinic acid,

di-(cyclohexyl)-phosphonic acid,

di-(phenyl)-phosphinic acid,

di-(n-butyl)-phosphinic acid,

di-(n-oxytyl)-phosphinic acid,

phenyl-cyclohexyl-phosphinic acid, and

(3,5,5-trimethylhexyl)-(n-butyl)-phosphinic acid.

It is preferred to use di-(2-ethylhexyl)-phosphoric acid, di-octyl-phosphoric acid, di-lauryl-phosphoric acid or butyl-hexyl-phosphoric acid. For the extraction of zinc from a used spinning bath,

it is preferred to use di-(2-ethylhexyl)-phosphoric acid.

The alkyl phosphoric acid can be used in concentrated form if it has suitable physical properties at the operating temperature. However, it is preferred to use it diluted in water

- immiscible organic solvent, in which the alkyl phosphoric acid must be very easily soluble. An aliphatic or aromatic hydrocarbon which is non-viscous at the operating temperature can be used as a diluent, such as a petroleum fraction, which from a safety point of view should have a very high flash point, an aromatic hydrocarbon such as benzene or a chlorinated hydrocarbon such as carbon tetrachloride. It is preferred to use a liquid consisting of 60 - 99% by weight diluent and

In 1 - 40% by weight of alkyl phosphoric acid. For cleaning a zinc-containing water solution obtained in the manufacture of rayon, it is preferred to use a liquid consisting of 70-80% by weight paraffin and 20-30% by weight di-(2-ethylhexyl) phosphoric acid.

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After the extraction process, the organic solution is brought

in contact, in a stripping or washing operation, but an aqueous solution to transfer the zinc from the organic solution to the aqueous solution. This can be done in several ways. To remove zinc from a viscose spinning bath, it is preferred

bringing the organic solution into contact with an aqueous, strong solution of sulfuric acid, preferably containing sulfuric acid in an amount of loo - l.ooo g/l. Zinc can also be washed out in the stripping step using an alkaline, aqueous solution, suitable sodium hydroxide, whereby zinc forms zinc cations in the alkaline solution. Another possibility is to wash out the zinc using an ammonia-containing aqueous solution, whereby zinc forms zinc ammonium ions. The zinc can also be washed out of the organic solution using an essentially neutral aqueous solution, for example a calcium chloride solution.

If an aqueous solution is to be treated, which, in addition to zinc, contains iron in significant quantities, it is desirable to remove zinc and iron from the organic solution in selective ...<y>ash steps, so that iron is separated from the zinc. This is achieved according to the invention by washing the organic solution. with an aqueous sulfuric acid solution in two steps. The first step comprises a washing with a solution containing free sulfuric acid in an amount of 5o - 2oo g/l, which results in zinc being completely transferred to the washing solution. The second step comprises washing with a solution containing free sulfuric acid in an amount of 5oo - 1.ooo g/l, which leads to . the iron is completely transferred to the washing solution. Slnk sulfate is preferably separated from the washing solution from the first stage by crystallization and the solution is then fed back to the washing vat. Iron sulfate is preferably separated from the washing solution from the second step in the same way.

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! In what follows, the invention will be further explained with reference to the attached drawings. Fig. 1 schematically shows a plant for cleaning a zinc-containing aqueous solution obtained during the manufacture of rayon. Fig. 2 is a McCabe-Thiele diagram and shows the equilibrium contents of zinc i

a liquid mixture in which the organic phase is di-(2-ethylhexyl)phosphoric acid in paraffin and the aqueous phase is an aqueous solution obtained in the manufacture of rayon. Figures 3-5 show how the distribution of zinc between the two phases depends on the temperature, see fig. 3, of the content of di-(2-ethylhexyl)phosphoric acid in the organic phase, see fig. 4, and of the content of sulfuric acid in the aqueous phase, see fig. 5.

Fig. 6 shows a plant for removing zinc from a solution which also contains iron.

The plant according to fig. 1 includes 4 appliances 1-4

for contact between and separation of an organic solution and an aqueous solution. The devices are of the mixing-trap type and comprise, see device 1, a mixing chamber 5, in which the two liquid phases are mixed by means of a stirring device 6, and a separation chamber 7, 8, in which the two liquid phases can be separated as a result of their different specific weights , as the lighter

the phase (generally the organic phase) is collected in the upper part 7, and the heavier phase is collected in the lower part 8.

The first two devices 1 and 2 form a two-stage extraction section, in which the zinc-containing aqueous solution is freed from zinc by means of countercurrent extraction with a solution of di-(2-ethylhexyl)phosphoric acid in paraffin. The third apparatus 3 forms the stripping step, in which the zinc in the organic solution is transferred to a strong sulfuric acid solution. The fourth apparatus 4 forms a washing step, in which a partial stream of the organic solution is continuously washed with alkali. The plant works in the following way: The zinc-containing solution from the rayon production is fed through a pipe line 9 to the mixing chamber in the second device 2 and mixed with the organic solution from the separation chamber in the first device 1.

i The two liquid phases are allowed to separate in the separation chamber in the second apparatus 2. The water phase is fed via the pipe line lo to the mixing chamber in the first apparatus 1, where it is mixed with an organic solution that is supplied through the pipe line 11

from the stripping step 3. After separation in the separation chamber 7,8,

the organic phase is fed to the mixing chamber in the second apparatus 2, while the water phase, which is now practically free of zinc, is drawn out through the rudder line 12.

The organic solution, which now contains zinc from the second separation chamber of the apparatus 2, is fed through a pipeline 13 to the mixing chamber in the third apparatus 3, where it is mixed with a strong sulfuric acid solution added through the pipeline 14. For practical reasons, the sulfuric acid solution can be concentrated sulfuric acid diluted with the spinning bath. After separation, the aqueous solution is fed out via the rudder line 15 and now contains zinc in the form of zinc sulphate. If desired, some of the aqueous solution containing zinc sulphate can be fed back to the mixing chamber through the rudder line 16. The organic solution, from which zinc has been removed, is withdrawn through the rudder line 17 and most of the solution is fed through the rudder line

11 to the extraction apparatus 1. A small part of the organic

solution is fed to the device 4, in which it is washed with a strongly alkaline solution supplied through pipe line 18. After separation, the alkaline solution is drawn out via pipe line 19, while the organic solution is fed back to the main stream

in the rudder line 11. Part of the extracted alkaline solution can be recycled to the mixing chamber through the rudder line 33.

A plant according to fig. 1 includes two extraction steps 1,2 in the extraction section. In certain cases one step may be sufficient, while in other cases three or more steps may be necessary. The number of steps required can be deduced from the McCabe-Thiele diagram shown in Fig. 2, in which the abscissa represents the zinc content in the aqueous solution and the ordinate represents the zinc content in the organic solution in equilibrium with the corresponding zinc content in the water. In the diagram according to fig.2, the zinc contents are expressed in grams of zinc per liter of solution. The line 2o is the equilibrium curve. Line 21 is

IN

jen working line, whose slope corresponds to the ratio H/L, i.e. the ratio between the flow of the heavier phase, i.e. the aqueous solution, and the flow of the lighter phase, i.e. the organic solution. In fig. 2 is H/L = o.5. It is preferred that the organic solution forms a continuous phase in the mixture, which is usually the case when the flow ratio H/L in

the mixing chamber is less than 1. The zinc content of the water solution that is introduced is represented by point 22 in fig. 2. Lines 22 - 23 (vertical), 23 - 24 (horizontal), 24 - 25 (vertical)

and its extension to point 26 is included. Point 26 represents the zinc content in the water phase, which is fed out after an extraction step. In the same way, the zinc content after two extraction steps can be obtained using lines 25 - 27 and 27 - 28.

The extraction can be carried out at normal temperature, but an increase in temperature up to 6o°C is preferred. The extraction is temperature-dependent as shown in curve 3o in fig..3. The abscissa represents the temperature in °C and the ordinate represents the distribution, D, of zinc between the two phases, i.e. the ratio between the zinc content in the organic solution and the zinc content in the aqueous solution. It can be seen that the D value rises

with rising temperature, i.e. that larger amounts of zinc are found in the organic solution.

The D value is also dependent on alkyl phosphoric acid in the organic solution. The curve 31 in fig. 4 shows how the D value

increases with increasing content of di-(2-ethylhexyl)-phosphoric acid.

Theoretically, the lowest required alkylphosphoric acid should be equivalent to the amount of zinc in the water solution, and it should be noted that the formed zinc compound is best stoichiometrically of zinc and alkylphosphoric acid in the ratio 1:2.

The extraction is also dependent on the sulfuric acid content of the water solution. The curve 32 in fig. shows this dependence for the extraction of zinc with di-(2-ethylhexyl)-phosphoric acid in paraffin. The abscissa represents the sulfuric acid content in g/l and the ordinate represents the D value. An increased sulfuric acid content results in a decreasing D value. In order to achieve a reasonable number

1 jet extraction step at 35 oC is preferred to work with

a water solution containing less than log sulfuric acid per litres.

As a consequence of the extraction in apparatuses 1 and 2, is almost

all the zinc in the zinc-containing aqueous solution, from the rayon manufacture, transferred to an organic solution, in which the zinc is present in the form of a compound of zinc and alkyl phosphoric acid. The zinc is recovered from the organic solution in the stripping step 3 by washing with a strongly acidic solution. Sulfuric acid is preferred for this washing, as the zinc sulphate formed can be fed directly back to the rayon spinning bath. The sulfuric acid solution should preferably contain loo - l.ooo grams of sulfuric acid per liter of solution, for example a concentration of 8oo g/l. The high acid content results in a splitting of the zinc compound, which leads to a very low D value and thus most of the zinc is transferred to the sulfuric acid solution. Due to the very low D value, it is possible to work with a very small flow of sulfuric acid and thus achieve a very high zinc content. For example, from a zinc content of 0.2 g/l in the soluble water solution, 15 g/l zinc can be obtained in the sulfuric acid solution. With a small flow of sulfuric acid, it can be difficult to mix the acid carefully with the organic solution. In this case, part of the acid solution from the separation chamber can be allowed to recirculate through the rudder line 16.

As mentioned above, the acidic zinc sulfate solution can be obtained

from the stripping step 3, is returned directly to the spinning bath. Alternatively, the zinc sulfate can be recovered from the solution, suitably by cooling the solution. A high sulfuric acid content reduces the solubility of zinc sulphate and thus promotes crystallization. A sulfuric acid content of approx. 6oo g/l is preferred. With a zinc content of 8o g/l, crystallization of zinc sulphate is achieved by cooling to approx. lo°C, resp;; zinc metal can be recovered from the solution by electrolysis. In this case, a sulfuric acid content of approx. 2oo g/l preferred to enable

c make a zinc content up to 12o g/l.

I The content of certain pollutants can gradually increase in the organic solution that circulates in the process. These can be metals other than zinc, for example iron or acid hydrolysis products as a result of splitting alkylphosphorus. the acid. It has been found that the content of such contaminants can be kept at an acceptable level by washing, in batches or

Jcontinuously, of a certain amount of the organic solution

with an aqueous alkaline solution, for example a solution of sodium hydroxide, preferably at a concentration of 100 - 300 g/l. NaOH. In the plant according to fig. 1 is carried out

this washing continuously in the appliance 4. Only a small part

of the flow of the organic solution, for example l/loo

-'—-JL/l.ooo is fed to the device 4, which can therefore be considerably smaller than the devices 1-3. During the alkaline washing step, the alkyl phosphoric acid is converted to the corresponding sodium salt. When the organic solution is fed back to the extraction stage 1, 2, this sodium salt will neutralize some of the sulfuric acid in the aqueous solution, which is fed to the extraction stage. It is thus possible to regulate the acid content in the extraction step 1, 2 to a certain extent by means of the flow of organic solution to the washing step 4. The alkaline washing step is also useful for another reason. It has proven difficult to completely separate the aqueous phase, containing sulfuric acid, from the organic phase in stripping step 3. If

this small amount of very strong sulfuric acid in the form of small drops

—if the organic solution now receives the extraction step 1.2, the content of sulfuric acid in this step will increase, which leads to

poorer extraction. However, the sulfuric acid can be neutralized by appropriate regulation of the alkaline washing process.

When alkylphosphoric acid dissolved in paraffin is used, it has been found that the sodium salt formed by the alkylphosphoric acid during the alkaline washing step has a lower solubility in paraffin than the acid itself. The sodium salt can therefore be precipitated in the boundary layer between the organic phase and the water phase during the alkaline washing process. This can be avoided by increasing the solubility of the sodium salt in paraffin by adding an aliphatic alcohol, which is essentially insoluble in water, preferably one with at least 8 carbon atoms in the aliphatic chain. A fitting

in addition is o.1 - o.5 times the amount of the alkyl phosphoric acid.

For example, the addition of 5% by weight of dodecanol has proven to be very effective when the alkyl phosphoric acid content is 25% by weight of di-(2-ethylhexyl >-phosphoric acid dissolved in paraffin).

The plant shown in fig. 6 contains 3 devices 61 - 63, in which an aqueous solution is brought into contact with an organic solution, the two solutions being essentially immiscible with each other and thus forming two phases. The devices are of the known mixing-trap type and comprise a mixing chamber 64, in which the two liquid phases are mixed by means of a stirring device 78, as well as a separation chamber 65, 66, in which the two phases are allowed to separate as a result of their different specific weights.

The first apparatus 61 receives an acidic sulphate solution via the rudder line 67. The solution contains zinc and iron. It can, for example, be a solution obtained by leaching iron-containing zinc ore with sulfuric acid. The solution is mixed in chamber 64 with a solution of a dialkyl phosphoric acid in paraffin, supplied through pipe line 68. The two phases are separated in chambers 5,6. The heavier phase, i.e. the aqueous phase, is practically free. for zinc and iron and is drawn off through pipe line 69. The lighter phase, i.e. the organic phase, is fed through pipe line 70 to a mixing chamber in apparatus 62, where it is mixed with an aqueous solution of sulfuric acid, supplied through pipe line 73. aqueous solution contains free sulfuric acid in an amount of 5o - 2oo g/l, preferably 8o - 12o g/l. Zinc, but not iron, is extracted from the sulfuric acid solution. This solution is now separated from the organic phase and fed through the rudder line 71 to a crystallization apparatus 72, in which part of the zinc sulfate is separated by crystallization in a known manner. The mother liquor is fed back to the washing apparatus 72 through a pipe line 73. The desired sulfuric acid concentration is maintained by adding sulfuric acid before or after the crystallization process.

The organic phase from the apparatus 62 is fed through a pipe line 74 to the mixing chamber in the apparatus 63, in which. mixed with an aqueous solution of sulfuric acid, supplied through the rudder line 77.

The sulfuric acid solution contains a quantity of free sulfuric acid

of 5oo - l.ooo g/l, preferably 800g/l. The iron content in the organic phase is taken up by the sulfuric acid solution, which is separated from the organic phase and fed through pipe line 75 to a crystallization apparatus 76, in which part of the iron sulfate is separated by crystallization in a known manner. The mother liquid is fed back to the washing device 63 through the rudder line 77.

The desired sulfuric acid concentration is maintained by adding sulfuric acid before or after the crystallization process.

The organic solution, which is thus selectively freed of zinc and iron, is fed back from the washing apparatus 63 through the pipe line 68 to the apparatus 61 for renewed use in the extraction process.

EXAMPLE

In a plant of the type illustrated in fig. 1, the devices 1-3 each comprised a mixing chamber of 12 m 3 and a separation chamber

- on loo m 3 . Apparatus 4 comprised a mixing chamber of o 15o 1 and a separation chamber of 2 m 3. A zinc-containing, aqueous solution

was obtained from a rayon fiber production in a quantity of 2 m 3 /min.

This aqueous solution was diluted with water to double the volume. The zinc-containing, aqueous solution was therefore quickly extracted in a quantity of 4 m 3 /min. This solution contained 0.15 g/l of tin and 4.2 g/l of free sulfuric acid. An organic solution was circulated in the plant in a quantity of 8 m 3 /min. The solution consisted of 70% by weight paraffin, 25% by weight di-(2-ethylhexyl)-phosphoric acid and 5% by weight dodecanol. The temperatures of the liquids were 35°C.

In the extraction steps, the zinc content of the water solution was reduced from o.15 g/l to less than o.oo2^g/l. The organic solution from the extraction contained zinc in an amount of approx. o.o75 g/l. This solution was washed in a stripping step with an aqueous solution containing 800 g/l sulfuric acid, added at a rate of 40 l/min. In order to achieve a favorable phase relationship in the mixing chamber, approx..3 m 3/min. sulfuric acid solution re-circulated through the rudder line 16. The sulfuric acid solution, which was precipitated from the stripping step, contained 15 g/l zinc. This solution was quickly returned to the rayon manufacturing process.

!Alternatively, the strongly acidic aqueous solution containing zinc sulphate can be recycled to the stripping step after a certain amount of zinc has been removed in the form of zinc sulphate by crystallization after cooling. Of the organic solution from the stripping step, 80l/min. quickly to an alkaline washing process, to which a solution of 2oo g/l NaOH in water was also added in a quantity of 12 l/min. The alkaline solution was circulated through the rudder line 33 in a quantity of 36 l/min.

Claims (3)

1. Method for removing zinc from an aqueous solution, characterized in that the aqueous solution is treated in an extraction zone with an organic solution containing an alkyl phosphoric acid, whereby zinc and the alkyl phosphoric acid form a chemical compound that is more soluble in an organic solvent and is thus extracted from the aqueous solution, after which the organic solution containing zinc in a stripping zone is brought into contact with an aqueous solution so that the chemical compound is split and zinc is transferred to the aqueous solution, after which the zinc-free organic solution is fed back for renewed extraction, either directly or after a certain amount of it has been freed from impurities. 2. Method according to claim 1, characterized in that the alkylphosphoric acid is a dialkylphosphoric acid with the general formula In which R is a branched or unbranched hydrocarbon group with at least 5 carbon atoms. 3. Process according to claim 2, characterized in that di-(2-ethylhexyl) phosphoric acid is used as dialkyl phosphoric acid. in 4. "Procedure according to" claim 2, characterized in that di-octyl phosphoric acid is used as dialkyl phosphoric acid. 5. Method according to claim 2, characterized in that di-lauryl phosphoric acid is used as dialkyl phosphoric acid. 6. Process according to claim 2, characterized in that butyl-hexyl-phosphoric acid is used as dialkyl phosphoric acid. 7. Procedure according to claim 1, characterized by the alkyl phosphoric acid is diluted with a diluent that is poorly soluble in water. 8. Method according to claim 7, characterized in that an aliphatic or aromatic hydrocarbon or a mixture thereof, e.g. a petroleum fraction with a boiling point interval similar to kerosene. 9. Method according to claim 1, characterized in that an aqueous solution with a pH value of less than 6 is used. 10 . Method according to claim 9, characterized in that sulfuric acid is added to the aqueous solution in an amount of less than 3o. g/l. 11. Method according to claim 1, characterized in that the organic solution containing zinc is washed with a strongly alkaline solution, preferably containing 5o - 5oo g/l NaOH, whereby zinc is transferred to the alkaline aqueous solution. 12. Procedure according to requirements. 1, characterized in that the organic solution containing zinc is washed with a strongly acidic aqueous solution, whereby zinc is transferred to the acidic, aqueous solution. 13. Method according to claim 12, characterized in that an acidic aqueous solution containing sulfuric acid is used in an amount of loo - l.ooo g/l solution. 14. Method according to claim 13, characterized in that the strongly acidic aqueous solution containing zinc is cooled for crystallization of zinc sulphate which is separated from the solution.. - 15. Method according to claim 14, characterized in that the stripping is carried out with an aqueous solution containing approx. 6oo g/l sulfuric acid and so that the acidic aqueous solution will contain approx. 8o g/l zinc, after which the solution is cooled; for crystallization of zinc sulphate. 16. Method according to claim 14, characterized in that the mother liquor from the crystallization is fed back for washing the organic solution containing zinc. 17. Method according to claims 1 or 12, characterized in that a partial stream of the organic solution is washed with an alkaline aqueous solution to remove metal contaminants and acidic hydrolysis products. 18. Method according to claim 17, characterized in that an organic solution is used containing an aliphatic alcohol which is poorly soluble in water, preferably in an amount of 0.1 - 0.5 times the amount of the alkyl phosphoric acid. 19. Method according to claim 1, characterized in that the temperature is kept in the range lo - 6o°C. 20 . Method according to claim 1 for the removal of zinc and iron from an aqueous solution, characterized in that the aqueous solution is brought into contact with an organic solution containing an alkylphosphoric acid, whereby zinc and iron are absorbed in the organic solution, after which this is brought in a first washing operation contact with the aqueous solution containing sulfuric acid in an amount of 5o - 2oo g/l, which leads to zinc but not iron being absorbed in the aqueous solution and the organic solution thus treated in a second washing operation being brought into contact with the aqueous solution containing sulfuric acid in an amount of 5oo - 1.ooo g/l, which causes iron to be absorbed by the aqueous solution, after which the organic solution is fed back to the extraction step. 21. Method according to claim 2o, characterized in that zinc sulfate is removed from the aqueous, sulfuric acid-containing solution, from the first operation, by crystallization, after which the mother liquor is fed back to the first washing operation. 22. Procedure according to claim 2o,. characterized in that iron sulfate is removed from the aqueous, sulfuric acid-containing solution from the second washing operation, by crystallization, after which the mother liquor is fed back to the second washing operation. f