CA1170839A

CA1170839A - Extraction process for the separation of a highly nickel-free cobalt product from aqueous solutions of cobalt and nickel

Info

Publication number: CA1170839A
Application number: CA000378353A
Authority: CA
Inventors: Bror G. Nyman; Stig-Erik Hultholm; Matti J. Hamalainen; Lars G. Jansson; Tom O. Niemi
Original assignee: Outokumpu Oyj
Current assignee: Outokumpu Oyj
Priority date: 1980-05-27
Filing date: 1981-05-26
Publication date: 1984-07-17
Also published as: FI63442B; BE888888A; FI801709A; DE3119147C2; DE3119147A1; BR8103269A; SE453923B; CU35466A; PT73049A; MA19155A1; PT73049B; FR2483464A1; NO811782L; NO159733C; SE8103283L; JPS57123834A; ZW12081A1; NO159733B; ZA813377B; FI63442C

Abstract

ABSTRACT OF THE DISCLOSURE

A process for the separation of cobalt from magnesium and nickel by extraction is disclosed in which a magnesium-bearing aqueous solution which contains cobalt and nickel is contacted with an organic solution which contains di-(alkyl)-phosphoric acid, alkylphosphonic acid monoalkyl ester or alkylphosphinic acid monoalkyl ester under conditions in which the distribution co-efficient of cobalt is at minimum 1.5 times higher than the distribution coefficient of magnesium and at the same time at minimum 30 times higher than the distribution coefficient of nickel, in which case the cobalt/magnesium ratio, prior to the extraction or during the extraction in the aqueous solution is adjusted to between 0.2 and 2.

Description

1 1 7~839 Outokumpu Oy, Outokumpu An extraction process for the separation of a highly nickel-free cobalt product from aqueous solutions of cobalt and nickel The present invention relates to a process for the separation of cobalt and nickel by extraction from magnesium-bearing aqueous solutions of these metals. Magnesium is either intentionally leached from the cobalt/nickel ore or concentrate, if the ore or concentrate contains magnesium, and is not removed during the solution purification prior to the extraction or, where necessary, magnesium is added prior to or during the extraction. The object of the invention is the recovery of cobalt with a good yield from cobalt- and nickel-bearing solutions as a product with a high cobalt/nickel weight ratio.
The object is to extract more than 99 ~ of the cobalt and at the same time to increase the cobalt/nickel ratio of the recovered cobalt product to 1000-20,000 fold, as compared with the respective metal ratio in the feed. Owing to the improved separation, the process can also be applied to the removal of cobalt from solutions in which the niclcel/cobalt ratio is high, e.g. the solutions of nickel processes in which laterite is treated and in which the nickel/cobalt weight ratio may be over 1000 .
~, I ~ ~û83~

Previous methods of solving the cobalt/nickel separation problem which is known to be difficult, include the use of strony chloride solutions to bring the cobalt to a chlorido-cobaltate form or the use of ammoniacal solutions for the oxidation of the cobalt with air to a trivalent form; in both these cases a sufficient difference is achieved between the chemical behavior of cobalt and that of nickel in order to separate these metals by processes known to an expert in metallurgy. Such separation procedures are, however, a considerable burden on the process, unless it is also advantageous to leach the cobalt/nickel ore or concentrate in a chlorïde-based or respectively ammoniacal form.

For many alternative processes it would be more advantageous considering the whole to separate the cobalt as a cation from the cobalt/nickel solution in question, in which case the separation would be possible regardless of the type of anion in the solution. The anion could in this case be, for example, a sulfate, which would allow the use of sulfuric acid as an advantageous leaching chemical. It is suggested in U.S. Patent 3,399,055 that cobalt can be separated from nickel by using organo-phosphoric acid in its alkali or ammonium salt form as the extractant. The ammonium salt of di-(2 ethyl-hexyl) phosphoric acid is mentioned as a possible salt; this salt serves as a cation exchanger as it binds the metal being extracted and yields an equlvalent amount of ammonium ions into the aqueous solution.

In Finnish Patent 51,110 it is shown that cobalt/nickel separa-tion is improved by using an organic phosphoric acid as the extractant, if magnesium is added at a minimum rate of 5 g/l to the cobalt/nickel solution before theextraction. Magnesium is coextracted with cobalt at a somewhat lower pH than nickel and, when the metal concentration in the extraction solution increases, magnesium replaces nickel, thereby forcing the nickel out from the extraction solution. The final outcome is an improved separation of cobalt and nickel. However, under the conditions presented, a large part of the added magnesium is extracted, together with the cobalt to be separated.
/

1 ~1 7~839 The use of a phosphoric acid ester as the extractant for separating cob-alt and nickel has also been disclosed. According to this method, cobalt is extr-acted at an elevated temperature~ 40C being the lower limit, while the concentra-tion of cobalt in the extraction solution is raised to between 11 and 16 g/l. In order to control the pH of the cobalt/nickel solution, the said phosphoric acid ester, di-(2-ethyl-hexyl) phosphoric acid, is used in its sodium salt form.
According to another method, alkyl phosphonic acid monoalkyl ester is used as the extractant for separating cobalt and nickel. By using the said extrac-tant - it is suggested that it be used in an ammonium salt form - it has been pos-sible to extract the cobalt more selectively from cobalt/nickel sulfate solutionsthan it is possible by using a corresponding di-alkyl phosphoric acid type extrac-tant.
According to the present invention there is provided a process for the separation of cobalt from magnesium and nickel by extraction, comprising contact-ing a magnesium-bearing aqueous solution which contains cobalt and nickel with an organic solution which contains di-(alkyl)-phosphoric acid, alkylphosphonic acid monoalkyl ester or alkylphosphinic acid monoalkyl es-ter; controlling the distribu-tion coefficient of cobalt such that it is at minimum 1.5 times higher than the distribution coefficient of magnesium and at -the same time at minimum 30 times higher than the distribution coefficient of nickel, and adjusting the cobalt to magnesium ratio, prior to the extraction or during the extraction, in the aqueous solution to between 0.2 and 2. In this way a considerable improvement of the deg-ree of separation of cobalt and nickel is achieved compared to the degree achieved by prior known methods.
According to the invention, cobalt is ex-tracted from magnesium-bearing solutions of cobalt and nickel under such conditions that at the same time as cob-alt is separa-ted from nickel it is also separated from most of the magnesium.

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1 17~839 Since it has been considered important that the cobalt can be extracted as a cat-ion regardless of the anions present in the cobalt/nickel solution, only such cat-ion exchangers can be used as have ~C ~ DM ~ NNi. In this case, D stands for the distribution coefficient of the metal in question, and it expresses for the metal under discussion the concentration ratio C /C in two solution phases which have been brought into an equilibrium. The notation org is an abbreviation for the 'organic phase' and aq for the 'aqueous phase', notation ~ meaning 'is greater than'.

.

I 1 7~83g An extractant can be considered usable for the purpose according to the present invention if for the extractant under discussion there are found technically possible conditions in which, when the pH of the cobalt/magnesium/nickel solution is raised, cobalt begins to be extracted at a higher rate than the magnesium, which is extracted at a higher rate than nickel. The separation coefficient of cobalt and magnesium, ScO Mg~ which means the ratio of the distribution coefficients of cobalt and magnesium, should be at minimum 1.5 in order that most of the magnesium remain unextracted in a multiple-stage technical countercurrent extraction of cobalt. A higher ScO Mg value would be more advantageous if the ScO Ni value were respectively higher. For example, if ScO Mg rose to 10, it would be advantageous for ScO Ni to rise to 100 or higher.

Usable extractants are, for example, di-(alkyl) phosphoric acids having the general structural formula R - O \ ~O
/ P \ or (1) R - O OH
a~.kyl phosphoric acid monoalkyl esters having the general formula R \ ~ O
/ P \ or (2) R - O OH
alkyl phosphine acid monoalkyl esters having the general formula ,~0 ~ OH (3) In the compounds presented above, the organic radicals R can be mutually similar or different alkyl groups which contain at minimum five carbon atoms in order that the compounds do not dissolve to a significant degree in aqueous solutions.
Correspondi.ng alicyclic or aromatic compounds which instead oE

1 J 7~839 one or both alkyl groups contain a cyclic hydrocarbon or respectively an aryl radical are usable under conditions in which DCo ~ DMg ~ DNi. It has not been considered appropriate to list tens of examples of each group of compounds, but to poin~
out that so far the most recommendable radical is 2-ethyl-1-hexyl-since generally available compounds having this structure have average or better-than-average properties as extractants.

The extractant used is diluted with an organic solvent, whereby the physical properties of the extraction solution produced are adjusted to suit the operating temperature in question. The diluents used can be aliphatic, alicyclic, aromatic or chlorated hydrocarbons the viscosity of which is so low that it does not disaduantageously slow down the separation of the mixed solution phases after the extraction contact, and the flash point of which is safely lower than the operating temperature used. Hydrocarbons which are distilled between 190 and 250 C and the viscosity of which at room temperature is of the order of 1-2 cP are suitable for the purpose. Often it is also necessary to add, at 0.5-15 %
by volume, some agent which lowers the emulsifying tendency.
~igher alcohols such as isodecanol and dodecanols, as well as organic phosphates and phosphonates such as tri-n~butyl phosphate and tri-n-butyl phosphonate, act as such agents.

The most advantageous procedure is to separate cobalt from a cobalt/magnesiurn/nickel solution in which the cobalt/magnesium weight ratio is 1-2. Even 1 g/l magnesium clearly improves the cobalt/nickel separation, although usually it is more advisable to use magnesium concentrations above 3 g/l. Owing to the magnesium, the cobalt/nickel ratio in the separated cobalt product easily increases ten-fold, as compared with what i-t would be if cobalt were separated from a similar solution which contains practically no magnesium.

Usually cobalt/nickel ores and concentrates do not contain soluble magnesium to such a degree that their leaching would directly produce a solution with a sufficient magnesium concentration for the separation according to the present 6 1 ~ 7~83~

invention. The magnesium concentration can be advantageously increased by using a suitable amount of magnesium oxide as a neutralizing agent either during the cobalt extraction or during some preceding process stage. Alternatively, the extractant fed to the cobalt separation can be partly brought to a magnesiun salt form in advance, in which case magnesium is required respectively in a lower amount or not at all in the cobalt~nicke]
solution of the process in question.

By carrying out the separation of cobalt and nickel by using the magnesium addition according to the present invention, a cobalt product can be easily separated which has a ~ickel content of less than 0~1 ~ of the cobalt. At best, the separation coefficient ScO Ni rises to between 100 and 500, in which case it is possible, when necessary, to reduce the nickel content in the cobalt product to below 0.01 %. To be successful, the cobalt/nickel separation does not require as precise monitoring of the separation conditions as does the cobalt/magnesium separation taken as a simultaneous objective.
The latter determines the pH and the solution flow volumes at which the separation in question is to be carried out. The cobalt/magnesium weight ratio being 1-2, the concentration of nickel in the solution has practically no effect on the extraction of cobalt. When the cobalt/magnesium weight ratio in the cobalt product settles between 50 and 100, the respective cobalt/nicke:L weight ratio increases; according to test results, generally to between 5000 and 30,000.

On an industrial scale, the separation of cobalt from magnesium, and at the same time from nickel, is carried out according to the known countercurrent extraction principle by feeding the said cobalt/magnesium/nickel solution countercurrently in rela-tion to the extraction solution and suitably in contact with it. The extraction solution is thereafter directed in a similar countercurrent contact with the washin~ solution, the aim being to remove most of the extracted magnesium and nic~el from the extraction solution before the solution, in a countercurrent contact with an acid solution, is caused to yield `` I ~ 7Q839 the cobalt into the said acid solution .It is expedient to raise the pH in part of the obtained re-extraction solution to between 3.5 and 5.0 and to use this neutralized solution as the washing solution for the purification of the extraction solution recommended above. It is advantageous to combine the thereby obtained magnesium- and nickel-bearing solution with the cobalt/magnesium/nickel solution fed to the cobalt extraction.
The remainder of the re-extraction solution is treated for the recovery of cobalt by known hydrometallurgical methods.

The most recommendable type of extraction apparatus is a mixer-settler, which, in addition to an expedient solution/solu-tion contact, provides good means of controlling the separation of cobalt from magnesium and at the same time also from nickel.
By using the pH control procedure described in Finnish Patent 49 185, which is based on direct batching of the neutralizing agent into the mixer on the basis of measurement impulses given by the pH electrodes located in the dispersion in the mixer, the said metal separations can be controlled in the desired manner by selecting suitable pH reference values for those mixer-settler cells into which neutralizing agent must be added. Direct pH control of this type improves the controllabilit of the separation process in question, and at the same time the extraction agent can be used in the process in the state in which the extractant is after the re-extraction carried out by means of an acid solution. The extractant need not be brought to a form of a sodium or ammonium salt, as required by the separation processes described above. According to a preferred separation procedure, the pH of the aqueous solution is raised to between 4.5 and 5.5 in the last extraction cell, into which the extraction solution is directed after the re-extraction. The alkali used for pxeventing the falling of the pH can be a suitable neutralizing chemical such as sodium hydroxide solution, sodium carbonate solution or slurry, ammonia water or gas, or a slurry of calcium hydroxide or magnesium oxide. By similar unidirectional pH control, the falling of the pH below the reference values is also prevented during the second to last extraction stage and, when necessary, during the third to last I J 7~839 extraction stage. During the other extraction stages, in which the falling of the pH is no longer sharp, pH control is usually not required at all. When necessary, the pH can be raised during one or two washing stages in order to improve the separa~ion result by using similar unidirectional pH control. It is preferable to allow the pH of the aqueous solution to ~all freely to a value close to or slightly below pH 4.0, whereby-an excessive increase in the viscosity of the extraction solution can best be avoided.

When mixer-settler cells are used, the viscosity of the extraction solution must not increase to values of several tens of centipoise, since this would disadvantageously slow down the separation of the phases. Also, during the wash, in which the viscosity of the extraction solution tends to increase the most, the viscosity can be maintained below 15 cP, if the pH
is allowed to fall freely during the first extraction stages and the washing stages. A sufficient washing effect is achieved by using a high-cobalt solution with a cobalt concentration of over 50 g/l as the washing solution.

According to one alternative, the re-extraction is carried out without pH control by using as the re-extraction solution an acid solution which contains sulfuric acic~ 100-200 g/l and by using between the extraction solution and the re-extraction solution a phase ratio suitable for this concentration~ It is also possible to use pH control for controlling the re-extraction, in which case a relatively strong acid solution is batched to at least one re-extraction stage. In this case, part of the formed re-extraction solution, together with a water addition, is returned to the re-extraction in order to improve the con-tact between the solutions to be mixed during the re-extraction.

pH control can also be used for the selective re-extraction of cobalt in order to produce a purer cobalt product in cases in which cobalt is to be separated from solutions which, in spite of possible solution purification, contain low concentrations i 1 7Q~39 of metals extractable at a pH lower than cobalt, such as iron, molybdenum, zinc, aluminum, lead, calcium, manganese or copper.
Such re-extraction, which separates cobalt from the said metals, is carried out by using pH reference values between 2.0 and 3.5 pH. A more precise value is determined on the basis of the separation in question.

The invention is described below in greater detail with the aid of examples.

Example 1 A series of extraction experiments were carried out by using as the extractant a di-(2-ethyl-hexyl) phosphoric acid solution, by mixing this solution with a solution which contained cobalt and nickel, or cobalt, nickel and magnesium. In the individual extraction experiments, the pH of the sulfate solutions was raised to 3.5-5.5 either at room temperature or at an elevated temperature of 55 C. The metal concentrations in one sulfate solution were Co 11.0 g/l and Ni 10.5 g/l, and those in the other were Co 11.0 g/l, Ni 10.2 g/l and Mg 10.2 g/l. The composition of the extraction solution used was di-(2-ethyl-hexyl phosphoric acid 20 % by vol., tri-n-butyl phosphate 5.0 % by vol. and kerosine, which contained aromatics 20 ~ by vol., 75 %
by vol. The mixing ratio Vorg/Vaq before the raising of the pH
was 1.33. An alkali solution containing NaOH 120 g/l was used for raising the pH. The pH of the sulfate solution in question was raised in steps, the mixing period being 10 minutes after each raising before a sample was taken from the mixed dispersion for a chemical analysis.

The results compiled in Table 1 show that the effect of the magnesium addition on the separation of cobalt and nickel is strongly dependent on the temperature. At room temperature, C, a magnesium addition corresponding to magnesium 10.2 g/l raises the separation coefficient SCO Ni from 1.6-2.5 to

2.1-3.2, the corresponding raise at 55 C being from 5.9-8.0 to 10-98. The corresponding raise in the separation ~oefficient SCo Mg is from 0.39-1.6 to 0.80-1.72. It can be observed that the addition of magnesium is especially advantageous at an elevated temperature when the pH is raised to between 5.0 and 5.5. Thereby the separation of cobalt from magnesium is also improved to such a degree that part of the magnesium remains unextracted in a multiple-stage countercurrent separation of cobalt and nickel.

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Example 2 The effect oE magnesium on the separation o~ cobalt and nickel when the extraction solution used consists of 2-ethyl-hexyl-phosphonic acid mono-2-ethyl-hexyl ester 20 % by vol., tri-n-butyl phosphate 5 % by vol., and aliphatic kerosine 75 % by vol. was shown by extracting various amounts of cobalt from magnesium-bearing solutions of nickel sulfate. The concentrationC
of metals in the sulfate solutions used in the extraction experiments were: cobalt lO.0-10.3 g/l, nickel 9.5-9.8 g/l, and magnesium 0-18.6 g/l. The mixing ratio Vorg/Vaq was 1.33, when the raising of the pH of the metal sulfate solution in question was started in steps by means of an alkali solution containing sodium hydroxide 120 g/l. After the pH had risen to the reference value, mixing was continued for lO minutes before a sample of the dispersion was taken for a metal analysis of the phases, and the raising of the pH was continued until the next reference value was reached. The experiments were performed at room temperature.

An examination of the results shown in Table 2 show that a relatively small magnesium addition, 4.5 g/l, suffices to raise the separation coefficient SCO Ni from between 61 and 74 to between 105 and 190 within that pH range, 4.7-5.5, within which the extraction of cobalt is increased, the highest magnesium addition, 9.8 g/l, raising this separation coefficient to between 135 and 450. A separation coefficient above lO0 between cobalt and nickel creates good conditions for distinct extraction of cobalt in a multiple-stage technical counter-current separation process. Simultaneously, satisfactory separation between cobalt and magnesium is achieved, since the separation coefficient SCO Mg surprisingly proved to be clearly higher than when using a di-(2-ethyl-hexyl)-phosphoric acid solution as the extraction solution. SCO Mg values of 7.6-9.9 allow with relative ease an increase to between 50 and 100 in the cobalt/magnesium weight ratio in the separated cobalt product.

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The extracti.on solution consisted of 2-ethyl-hexyl-phosphonic acid mono-2-ethyl-hexyl ester 20 ~ by vol., tri-n-butyl phosphat~
5 % by vol. and kerosine 75 ~ by vol., the first-mentioned constituent in acid form being directed to the extractio~n directly from the re-extraction. The pH was maintained at reference value 5.0 during stage U5 and at reference value 4.9 and respectively 4.7 during stages ~4 and U3 by using the pH
measurement impulses obtained from the mixer dispersions in ~uestion for automatic dispen~.lng of the neutralizing agent. The neutralizing agent was an alkali solution which contained sodium hydroxide 200 g/l. The pH of the other stages was allowed to fall freely.

The washing solution used was a re-extraction solution neutralized to a pH of 4.0 by means of sodium hydroxide, the analysis of the solution being cobalt 81.0 g/l, nickel 0.002 g/l and magnesium 1.09 g/l. During the wash, the concentration of cobalt in the solution decreased to 25.3 g/l, its nickel concentration increasing to 0.43 g/l and its magnesium concentration increasing to 27.4 g/l, the pH remaining unchanged.

The used washing solution was combined with the cobalt/magnesium/
nickel solution fed to the extraction. When the operation was started from a ~eed which contained cobalt 7.0 g/l, nickel 4.46 g/l and magnesium 6.0 g/l, the product was a solution which contained cobalt more than 100 g/l and in which the cobalt~nickel ratio was 25,000 and the cobalt/magnesium ratio 60, as well as a nickel solution which contained cobalt 0.025 g/1 and in which the nickel/cobalt ratio was 160. By this operating procedure, the dynamic viscosity of the extraction solution remained below 12 cP even during the wash.

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~ O -~ - ~ - ' ~ ~ 3a I 1 7~839 Exam~le 4 In a series of extraction experiments in which the extraction solution consisted of 2-ethyl-hexyl-phosphonic acid mono-2-ethyl-hexyl ester 20 % by vol., tri-n-bityl phosphate 5 % by vol. and kerosine 75 % by vol., it was observed how other metals which are extracted at a higher rate than cobalt are distributed, as a function of the pH, between the said organic phase and the metal-bearing aqueous solution brought into a mixing contact with the organic phase. The pH dependence of the distribution coefficient D obtained as the result as regards the metals in question is shown in Figure 1. This figure shows that the extraction equilibrium or respectively the re-extraction equilibrium within a pH range of 2.0-3.5 prevents most of the cobalt from being extracted or respectively allows the re-extraction of most of the cobalt, while the metals in question pass to a considerable degree into the extraction solution or respectively remain in this phase. The degree of purity of the obtained cobalt product can be raised by using selective re-extraction. On the basis of the result presented, an expert in the field can calculate the pH which is advantageou~ in a given case for the $elective re-extraction of~ cobalt. For example, at a pH of 2.5-3.0 the cobalt can be separated from calcium, and from metals which are extracted at a higher rate than is this metal.

Claims

WHAT IS CLAIMED IS:

1. A process for the separation of cobalt from magnesium and nickel by extraction, comprising contacting a magnesium-bearing aqueous solution which contains cobalt and nickel with an organic solution which contains di-(alkyl)-phosphoric acid, alkylphosphonic acid monoalkyl ester or alkylphosphinic acid monoalkyl ester; controlling the distribution coefficient of cobalt such that it is at minimum 1.5 times higher than the dis-tribution coefficient of magnesium and at the same time at mini-mum 30 times higher than the distribution coefficient of nickel, and adjusting the cobalt to magnesium ratio, prior to the ex-traction or during the extraction, in the aqueous solution to between 0.2 and 2.

2. A process according to Claim 1, in which the extractant is di-(2-ethyl-hexyl)-phosphoric acid and the temperature 40-80°C.

3. A process according to Claim 1, in which the temperature is 50-60°C.

4. A process according to Claim 2, in which the di-(2-ethyl-hexyl)-phosphoric acid is diluted with a hydrocarbon which con-tains aromatics at minimum 20 % by weight.

5. A process according to Claim 1, in which the organic solution contains tri-n-butyl-phosphate or tri-n-butyl-phosphon-ate 5-20 % by volume.

6. A process according to Claim 1, in which the pH of the aqueous solution is 4.5-5.5.

7. A process according to Claim 1, in which the extractant is 2-ethyl-hexyl-phosphonic acid mono-2-ethyl-hexyl ester.

8. A process according to Claim 1, in which the extractant is 2-ethyl-hexyl-phosphinic acid mono-2-ethyl-hexyl ester.

9. A process according to Claim 7 or 8, in which the tem-perature is 20-80°C.

10. A process according to Claim 7, in which the extractant is diluted with an organic hydrocarbon.

11. A process according to Claim 7, in which the organic solution contains tri-n-butyl phosphate or tri-n-butyl phos-phonate 0-20 % by volume.

12. A process according to Claim 7, in which the pH of the aqueous solution is 4.0-6Ø

13. A process according to Claim 1, comprising passing the extraction solution in a countercurrent contact with a magnesium-bearing aqueous solution which contains cobalt and nickel and thereafter in a countercurrent washing contact with the neutral-ized part of the cobalt solution obtained in subsequent counter-current contact between the extraction solution and the acid solution, and maintaining, in order to lower the viscosity of the extraction solution, during the washing contact the pH of the cobalt solution 0-1.0 pH unit lower than the pH of the magnesium-bearing aqueous solution of cobalt and nickel during the extraction contact.

14. A process according to Claim 13, in which the pH of the magnesium-bearing solution which contains cobalt and nickel is 5.0-5.5 during the extraction contact and the pH of the cobalt solution is 4.5-5.0 during the washing contact.

15. A process according to Claim 7, in which the pH of the magnesium-bearing aqueous solution which contains cobalt and nickel is 4.6-5.1 during the extraction contact and the pH of the cobalt solution is 4.1-4.6 during the washing contact.

16. A process according to claim 1, in which the concentration of magnesium in the aqueous solution which contains cobalt and nickel is increased during the leaching or solution purification, preceding the extraction of cobalt, by leaching magnesium-bearing raw materials or respectively by adding a soluble magnesium com-pound.

17. A process according to claim 16, in which the soluble magnesium compound is magnesium oxide.

18. A process according to claim 1, in which the concentration of magnesium in the aqueous solution which contains cobalt and nickel is increased during the extraction of cobalt.

19. A process according to claim 18, in which the extraction agent is used in an acid form and protons released during the cobalt extraction are neutralized by means of magnesium oxide.

20. A process according to claim 18, in which the extractant used for the extraction of cobalt is in part converted to a magnesium salt prior to the extrac-tion.

21. A process according to claim 14, 15 or 16, in which the extractant used is in an acid form and that, in order to neutralize protons released during the extraction, an alkali is added to at least the two last extraction stages.

22. A process according to claim 14, 15 or 16, in which the extractant used is in an acid form and that, in order to neutralize protons released during the extraction, sodium hydroxide or ammonia is added to at least the two last extrac-tion stages.

23. A process according to claim 13, in which the countercurrent contact between the extraction solution and the acid solution is effected at a pH of 2.0-3.5 in order to re-extract the cobalt selectively.

24. A process according to Claim 23, in which during the re-extraction, the cobalt is separated from at least one element selected from the group comprising iron, zinc, aluminum, lead, calcium, manganese and copper.