NO782342L - PROCEDURE FOR EXTRACTING NON-IRON METALS FROM AN IRONOUS SOLUTION - Google Patents

Description

The present invention relates to the recovery of nickel, cobalt, manganese, copper and zinc from ferrous solutions by solvent extraction with the participation of alkyl- or aryl-substituted phosphoric acids.

It is known that organic solutions of various substituted phosphoric acids in which one or two of the hydrogen atoms are substituted with alkyl or aryl radicals are suitable for solvent extraction of various non-ferrous metals from aqueous solutions. It is known that during the solvent extraction, hydrogen ions are released, and as the extraction is pH sensitive, the resulting lowering of the pH, if not counteracted, will stop it.

As a solution to this, it has been proposed to add the necessary amount of base before the extraction, either to the aqueous solution or to the organic phase, or to add it during the course of the extraction. The base can be added directly to the extraction container, or alternatively, part of the aqueous phase can be continuously withdrawn from the extraction container, neutralized, filtered if necessary and returned to the container, as described in US patent no. 3,479,378.

An extraction process of this type is particularly well suited for inclusion in an overall hydrometallurgical process for extracting non-ferrous metals from ores containing them. For example, the overall process may include acid leaching of the ore, solvent extraction of the non-ferrous metals from the leaching solution with subsequent recovery of the extracted metals from the solvent and finally electrolytic purification of the metal. However, problems are encountered when - which is almost always the case - the solution from which you want to extract nickel, for example, also contains some iron. The first difficulty this entails arises from the fact that if the conditions are present at any stage during the extraction, the iron may precipitate from the solution. When this happens, the precipitated material will tend to form a stable emulsion with the organic and the aqueous liquid. This emulsion, which in English-speaking countries is often called potash "crud", can make further extraction so slow that it becomes unacceptable for practical purposes. The other difficulty due to the presence of iron in the extraction container arises because the iron, under the conditions suitable for absorption of the non-ferrous metal in the extraction agent, will be absorbed in the extraction agent at least as easily as the non-ferrous metals. Once the iron is dissolved in the organic liquid,

is more difficult to recover than the non-ferrous metals. When, for example, the organic liquid is brought into contact with diluted mineral acid, which is common when it comes to recovering nickel from the liquid, this will not effectively remove the iron. It would therefore be necessary to carry out a further step which effectively removes the iron before the organic liquid is recycled for renewed use, otherwise the iron content will eventually increase so much that it has a detrimental effect on the solvent's ability to extract nickel.

For the above reasons, it is tacitly assumed in much of the literature in the area that. the aqueous-organic mixture in the solvent extraction does not contain iron. However, while such iron-free conditions can be achieved in laboratory experiments, they are difficult to achieve in a practical industrial process for treating leachates. Thus, a simple hydroxide precipitation can remove most of the iron from the aqueous solution before the solvent extraction, but generally traces of iron will remain. Furthermore, some iron may be introduced due to corrosion of the equipment containing the acidic, aqueous solution. Even when the amount of iron present in the aqueous solution fed to the solvent extraction step is small enough not to cause problems to begin with, the iron will accumulate in the organic phase, as described above, and eventually lead to the formation of "crud". This problem has been recognized by a number of previous researchers, and attempts to overcome it are described in US Patent No. 3,193,381 and No. 3,666,446.

The scheme described in US patent 3,193,381 consists in first carrying out an aqueous-organic solvent extraction under such conditions that only iron is removed from the aqueous phase; then the aqueous phase is treated with a base to precipitate nickel and cobalt as hydroxides, after which the resulting slurry is subjected to an aqueous-organic solvent extraction whereby the nickel and cobalt are dissolved. The iron removal step must be applied to the entire aqueous stream, as traces of iron would otherwise undoubtedly cause problems with emulsion formation ("crud") under the alkaline conditions used for nickel-cobalt extraction.

The method described in US patent

3,666,446, comprises extraction of the iron together with the non-ferrous metals, after which the enriched extractant is subjected to successive re-extractions. In the first re-extraction, the non-ferrous materials are recovered in a conventional manner, and in the second re-extraction, iron is removed using an organic chelating agent.

It will be understood that the treatment of either the entire aqueous stream or the entire organic stream for the removal of iron contributes significantly to the overall costs of the process, and it is an object of the present invention to reduce the costs as much as possible: According to the present invention a method is provided for extracting one or more of the non-ferrous metals nickel, cobalt, manganese, copper and zinc from an acidic aqueous solution containing iron in an amount of no more than 1 g/l characterized by the following steps: 1) the aqueous solution is brought into contact with an organic liquid comprising an organic-substituted phosphoric acid and an organic solvent therefor which is immiscible with water, base being added to the aqueous-organic mixture to maintain its pH at a predetermined value not exceeding 6 ,5, where the iron and the desired non-ferrous metal(s) are brought to be enriched in the organic liquid, and that the enriched organic liquid is called illes from the depleted aqueous solution; 2) the non-ferrous metal or metals are re-extracted from the organic liquid by bringing this into contact with an aqueous acidic solution, and the resulting ferrous organic liquid being separated from the non-ferrous metal-containing re-extraction solution, 3) the ferrous organic liquid is divided into a hdved stream and a smaller draw-off stream, 4) the draw-off stream is brought into contact with an aqueous iron removal solution selected from mineral acid solutions capable of replacing iron with protons in the draw-off stream , and solutions containing citric acid, alkali metal citrate or a carbohydrate capable of forming a chelate compound with the iron, and the ferrous solution is separated from the treated drain stream, and 5) the treated drain stream is combined with the main stream of ferrous organic liquid, and the combined stream is recycled for use in step (i) on a further quantity of the original aqueous solution.

It is a characteristic feature of the present method that the organic liquid that is recycled to the solvent-extraction step will contain some iron, as the iron removal is only carried out on the draw-off stream to maintain the concentration of iron at a predetermined level in the organic phase. The economic advantage of the method is due to the fact that only a small drain stream has to be subjected to the iron removal step. This drain stream can actually amount to as little as 10% by volume of the total ferrous organic liquid to be reused. The exact proportion of the organic liquid that is split off as a draw-off stream will depend on the iron content of the aqueous feed stream as well as the iron content that can be tolerated in the organic feed stream. It is generally preferred that the draining flow is between 10 and 25 volumes? of the total organic liquid.

It should be emphasized that the presence of iron in boats

the aqueous and organic phases in the solvent extraction vessel can only be tolerated without adverse effects on the extraction if another essential feature of the invention is observed, namely that in situ neutralization is used in the solvent extraction process to ensure acidic conditions for any time. It was found that emulsion formation ("crud") does not take place to a detectable extent at a pH lower than 6.5. If the pH rises above this value, even just for a time, however, an emulsion will form, and this will not dissolve easily. Thus, the method cannot be carried out by adding the base required for solvent extraction to the organic or the aqueous feed solution before the solution is introduced into the extraction vessel. It is essential to monitor the pH value in the extraction vessel and add the necessary base, which is preferably sodium hydroxide, so that the pH value is kept at the desired level.

The pH value to be used during the solvent extraction process will depend on several criteria. The first of these, already mentioned, is necessity

of keeping the pH below 6.5 with the aim of avoiding emulsion formation. Secondly, however, an excessively low pH will be unfavorable

for the uptake or extraction efficiency of the organic extractant, and for a given extractant and given metal to be extracted, a minimum pH can be determined which will ensure sufficient uptake or extraction. When more than one metal is present in the aqueous feed material, the choice of pH will of course depend on whether the aim is to extract several metals or, which is more common, to extract one of them selectively. An important application for the method according to the invention thus lies in the treatment of solutions containing both cobalt and nickel, where these are to be separated from each other by selective extraction of cobalt.

Another criterion that affects the choice of pH, within an acceptable range, is the viscosity of the organic liquid. It was found that upon neutralization of the organic liquid, a sharp increase in viscosity occurs when the pH is exceeded, which corresponds to approx. 90% conversion of the phosphoric acid into its salt form. For example, the viscosity can increase by as much as an order of magnitude when the pH changes from a value corresponding to 90% conversion of the acid to a value corresponding to 100% conversion. As a very viscous organic phase results in slower mass transfer and phase separation, it is preferred to use a pH that corresponds to a transfer of no more than 8 5-9 0%

of the phosphoric acid to the salt form. For the extractant di(2-ethylhexyl)-phosphoric acid (D-2-EHPA), this sets a preferred upper limit of approx. 5.0 for the pH to be maintained during the extraction, and the pH value is advantageously kept in the range 4.8-5.0.

The substituted phosphoric acid used in carrying out the present method can be any of the various organophosphoric acids which are known to be suitable for the extraction of metals such as nickel and cobalt from aqueous solutions. The best known of these are probably di(2-ethylhexyl)-phosphoric acid. Other suitable acids are, for example, de-octyl phosphoric acid, octylphenyl phosphoric acid, di(1-methylheptyl) phosphoric acid and dodecyl phosphoric acid. They can be represented by the general formula HRR'PO^, where R and R'

can be an alkyl, aryl, or alkaryl radical.

The solvent, by which is meant the diluent which forms part of the organic phase in the method according to the invention, can be any organic liquid which is immiscible with the aqueous phase and in which the extractant is at least partially soluble. The concentration of the extractant in the solvent is chosen according to the amount of metal to be extracted from the aqueous phase. The solvent used is preferably a solvent in which the solubility of the extractant is 20% or more. Typical solvents are aliphatic or aromatic hydrocarbons, such as kerosene, as well as alcohols such as isodecanol, and phenols such as paranonyl-phenol, or mixtures of such compounds. When the solvent consists of a mixture of organic liquids, one of the component liquids is often called a modifier, as one of its functions is to prevent the organic substances from splitting into two phases. In general, it is preferred to use diluents as solvents which do not make it necessary to use a modifier in order to avoid such a division of the organic phase. Particularly suitable diluents are, for example, the commercial solvents which are in the trade under the trade names "Escaid 100" (from Imperial Oii Ltd.) and "Kermac 470B" (Kerr McGee Corp.).

The aqueous and organic phases may have to be brought into contact with each other in more than one extraction step. In that case, it may be necessary, depending on the initial pH of the aqueous feed material, to regulate the pH by adding alkali to only some of the steps. The addition of alkali can conveniently be carried out automatically by means of

a pH meter-titrator connected to pH electrodes that are placed directly in the phase mixture.

After separation from the used aqueous solution, the organic material is processed for the recovery of nickel, cobalt, manganese, copper or zinc. When, as is usual, two of these metals, for example cobalt and nickel, are present in the organic material together with the iron, it will be desirable to remove one of these non-ferrous metals, for example nickel, by a washing operation, whereby what remains is an organic liquid containing mainly only iron and the other non-ferrous metal, for example cobalt. The latter is then removed from the organic liquid, leaving essentially only iron in the organic liquid. In this way, an aqueous washing liquid and an aqueous removal liquid or "stripping liquid" are obtained, from which the respective non-ferrous metals can be recovered separately, for example by electrolysis.

The washing and stripping operations for removing the non-ferrous metals can be carried out with aqueous solutions of the same or different compositions. As a rule, a 1 N sulfuric acid solution is used. When the first metal to be removed from the organic liquid is only present in small amounts therein, it will be appropriate to recycle the aqueous washing liquid to the main extraction step, where it is combined with a further aqueous feed solution, whereby the concentration of the relevant non-ferrous metal in this is increased. When using such a scheme, the recycled non-ferrous metal will be recovered from the raffinate, i.e. from the aqueous solution from which the iron and other non-ferrous metal have been extracted with an organic solvent.

The removal or stripping of the iron from the tapping stream can be carried out using a more concentrated mineral acid solution than that used for stripping the non-ferrous metal. For example, a hydrochloric acid solution with a strength of 4-6 N can be used for stripping the iron, provided that the organic solution does not contain a modifier. The mainly iron-free draining stream is then separated from the FeCl^-containing aqueous phase, and the former is ready for new use in combination with the main stream of organic liquid, which can contain up to several grams of iron per litres.

In an alternative method for stripping iron from the tapping stream, an aqueous solution of a complexing agent, such as an alkali metal citrate, is used as stripping liquid. This liquid can then be regenerated for repeated use by treatment with alkali and separation of the precipitate of trivalent iron. Other iron-removing reagents that can be used are, for example, saccharides and various polyhydric alcohols, such as mannitol, all of which are capable of forming chelate complexes with iron.

Theoretically, one could remove iron from an organic liquid, such as the above-mentioned tapping stream, by direct addition of a strong base. However, this would necessarily entail the precipitation of iron (III) hydroxide in the presence of the organic liquid, so that the subsequent phase separation required to provide an emulsion-free draw-off stream which can be recycled would be very difficult to achieve. Accordingly, the direct addition of base to the tap stream to remove iron therefrom is not used in the process according to the invention. Instead, it is a feature of the method according to the invention that during the metal extraction operation as well as the various metal stripping operations, neither the organic stream as a whole nor any part thereof is at any time subjected to alkaline pH conditions which can lead to emulsion formation ( "crud formation").

Some examples will now be given.

Example 1

This example concerns the separation of nickel and

cobalt from each other and from iron in an aqueous sulphate solution, which contained 6.5 g/l cobalt, 11.7 g/l nickel and 0.088 g/l

iron, with a 30 volume% solution of D-2-EHPA in the above-mentioned commercial solvent "Escaid 100" being used as the organic liquid.

A continuous extraction was carried out by bringing the aqueous phase and the organic phase (in the volume ratio 3:1) to flow countercurrently with each other through 5 mixing-separating vessels. the temperature of the phase mixture was maintained

at 60°C during the extraction, and in response to signals from pH electrodes immersed in the last mixer (seen in the direction of the water flow), alkali was automatically added to this mixer so that

a pH between 4.8 and 4.9 was maintained. The alkali used was a 36-50% by weight solution of sodium hydroxide in water.

The enriched organic mixture obtained from the above extraction was subjected to a washing operation to remove the small amounts of nickel extracted under the selected conditions. This was carried out in a three-step operation using a 1 M sulfuric acid solution, using an aqueous to organic phase ratio of 1:10. The wash liquor was recycled to the extraction step and combined with additional aqueous feed solution to be treated. In this way, the entire amount of nickel in the original starting material was finally collected in it. final raffinate, i.e. the aqueous stream which is withdrawn from the 5-stage extraction operation.

The washed organic liquid was then passed through

3 mixing-separating vessels in countercurrent with a 1 M sulfuric acid solution, using a ratio between aqueous and organic phase of 1:3, for the removal of cobalt.

The organic liquid, which now essentially only contained iron, was divided into two different streams. The main stream was recycled to the extraction stage, while the minor stream or draw-off stream, which amounted to only approx. 15 volume? of the treated organic liquid, was first freed of iron with an equal volume of a 0.5 M solution of sodium citrate. The fully purified organic liquor was then recycled to the extraction vessel and combined with the recycled ferrous main organic liquor stream. The ferrous citrate solution was treated with a 50% by weight sodium hydroxide solution so that its pH was raised to 10.5, whereby the iron was precipitated as ferric hydroxide. After filtering off the precipitate, the citrate solution could be used again for the extraction of iron without pH regulation, as it had had its pH reduced to approx. 5.0.

Analysis of the various aqueous and organic streams showed that: a) in the extraction step, all iron and cobalt had been transferred to the organic phase. Thus, the aqueous raffinate obtained from the extraction step contained less than 0.0003 g/l iron, and the nickel/cobalt ratio was 166:1. b) The enriched organic liquid obtained from the extraction step contained nickel in an amount corresponding to approx. 2% of the total amount of nickel in the original aqueous starting material. c) The washing operation removed all the nickel, but none of the iron from the organic liquid. d) The cobalt extraction did not remove any iron from the organic liquid. The aqueous cobalt extraction liquor contained 58 g/l cobalt and had a cobalt/nickel ratio of 10,280:1. It could therefore be used for the electrolytic extraction of a cobalt product of very high purity.

e) The effect of removing iron from the drawdown stream was to regulate the iron content of the organic liquid

which is added to the extraction step at approx. 1.5 g/l.

The process was thus carried out with aqueous and organic starting materials, both of which contained significant amounts of iron. These iron contents did not result in any emulsion formation in the circuit, nor did they have any detrimental effect on efficiency.

Example 2

In order to investigate the effect of an even higher iron content in the organic starting solution, a series of experiments were carried out in which an iron-free aqueous phase containing 8 g/l cobalt and 20 g/l nickel was brought into contact with different organic starting materials corresponding to those used in example 1, but with the exception that they contained between 1 and 12 g/l iron(III) ions. In each case, the extraction was carried out at 50°C using equal volumes of aqueous phase and organic phase, and sodium hydroxide was used to maintain a pH of 5.0 ± 0.05. Even with these high iron contents, no precipitation was observed, and in no case was there any change in the iron concentration in the organic liquid as a result of the extraction.

Example 3

The results in the present experiment show the efficiency of the removal of iron with citrate solution. An organic phase containing 1.3 g/l iron was contacted with an equal volume of 0.5 M citric acid solution at 65°C. A sodium hydroxide solution was added so that the pH was gradually raised to 5.0, and the liquids were analyzed at various time intervals. The results are shown in the table below.

It will thus be seen that at a pH of 5.0 or

higher, such a citrate solution will extract essentially all the iron in the organic draw-off stream.

Claims (6)

1. Process for extracting one or more of the non-ferrous metals nickel, cobalt, manganese and zinc from an acidic, aqueous solution containing iron in an amount of no more than 1 g/l, characterized by the following steps: 1) the aqueous solution is brought into contact with an organic liquid comprising an organic-substituted phosphoric acid and a water-immiscible organic solvent for this, with base being added to the aqueous-organic mixture to maintain its pH at a predetermined value which is not higher than 6.5, whereby the iron and the desired non-ferrous metal(s) are enriched in the organic liquid, and the organic liquid thus obtained is separated from the depleted aqueous solution; 2) the non-ferrous metal or metals are stripped from the enriched organic liquid by bringing this into contact with an aqueous acidic stripping solution, and the resulting ferrous organic liquid is separated from the stripping solution containing the non-ferrous metal or metals ; 3) the ferrous organic liquid is divided into a main flow and a minor drain flow; 4) the stripping stream is brought into contact with an aqueous iron-stripping solution selected from mineral acid solutions capable of replacing iron with protons in the stripping stream, and solutions containing citric acid, alkali metal citrate or a carbohydrate capable of forming a chelate compound with the iron , and the ferrous stripping solution is separated from the treated drain stream; 5) the treated drain stream is combined with the main stream of ferrous organic liquid, and the combined stream is recycled to carry out step (1) on a further quantity of the original aqueous solution. 2. Method according to claim 1, characterized in that the tapping flow constitutes between 10 and 25% by volume of the iron-containing organic liquid. 3. Method according to claim 1 or 2, characterized in that the substituted phosphoric acid is di(2-ethylhexyl) phosphoric acid. 4. Method according to claim 3, characterized in that the aqueous starting solution contains iron, nickel and cobalt. 5. Method according to claim 4, characterized in that the predetermined ph value is between 4.8 and 5.0, whereby the enriched organic liquid in step (1) contains most of the cobalt and only a small part of nickel present in the original aqueous starting solution. 6. Method according to claim 4 or 5, characterized in that the stripping in step (2) comprises a first operation in which nickel is stripped from the enriched organic liquid, and a second operation in which cobalt is stripped from the nickel-poor, enriched organic liquid .