WE CLAIM:
1. Process for preparing a high-purity nickel sulphate solution, comprising the
steps of:
i. providing an aqueous feed solution comprising nickel, cobalt, calcium and
magnesium;
ii. in a first solvent extraction circuit, extracting cobalt, calcium and, at least
in part, magnesium, and if present zinc, copper, cadmium and manganese, from said aqueous feed solution using a first organic phase comprising a first alkylphosphorus-based extractant (I) and a first diluent,
thereby obtaining an aqueous raffinate solution comprising nickel and a
residual magnesium content and a cobalt-rich and calcium containing organic phase;
iii. in a second solvent extraction circuit, extracting magnesium from said
aqueous raffinate solution using a second organic phase comprising a
second alkylphosphorus-based extractant (II) and a second diluent,
thereby obtaining a magnesium-depleted, high-purity aqueous nickel sulphate solution and a magnesium-enriched organic phase; and
iv. stripping said cobalt-rich organic phase obtained in step ii. with an aqueous solution comprising a mineral acid,
whereby said first solvent extraction circuit and said second solvent extraction
circuit operate at a different temperature and/or with a different extractant.
2. Process according to claim 1, whereby said mineral acid is hydrochloric acid.
3. Process according to any of claims 1 to 2, whereby nickel is scrubbed from said
cobalt-rich organic phase and/or from said magnesium-enriched organic phase.
4. Process according to claim3, whereby said iron and/or aluminium is removed by
precipitation in two or more precipitation steps, whereby a different precipitating
agent or a combination of precipitating agents may be used in each precipitation
step.
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5. Process according to any of claims 1 to 4, whereby said first and second alkylphosphorus-based extractant comprise an alkylphosphorus-based acid
and/or nickel salts thereof.
6. Process according to any of claims 1 to 5, whereby said second extractant (II)
has a higher selectivity for magnesium than said first extractant (I).
7. Process according to any of claims 1 to 6, whereby said first alkylphosphorusbased extractant (I) comprises an alkylphosphonic acid and/or nickel salts
thereof, and whereby said second alkylphosphorus-based extractant (II) comprises an alkylphosphinic acid and/or nickel salts thereof.
8. Process according any of claims 1 to 7, whereby said first and said second extractant are converted to their nickel salts, containing nickel with a concentration of at least 20% of the available extractant capacity.
9. Process according to claim 8, whereby said first and said second extractant are
converted to a nickel salt, and whereby said first and said second organic phase
comprise less than 2 g/L sodium.
10. Process according to any of claims 1 to 9, whereby said first and said second
organic phase provided in steps ii. and iii. comprise said first and said second
extractant, respectively, in an amount of 5 to 50 vol.%, relative to the total
volume of said solvents, and said diluents in an amount of 50 to 95 vol.%,
relative to the total volume of said solvents.