[JustAnotherPerson95]

So I have a sludge sample which contains ~18% Ni, ~19% Cr, ~6% Cu, ~4% Fe and ~0.26% Al (According to ICP-OES analysis) after leaching with H₂SO₄, and basically I only want the Ni and Cu, so I need to separate the Cr, Al and Fe ions from the solution (Which is originally black-brown).

Initially I tried adding NaOH until it reaches about pH 12 to separate Cr and Al from the other metals, under the assumption that Cr and Al ions will remain in the solution due to their amphoteric nature while Cu/Ni/Fe remain as the precipitate. However, upon ICP-OES analysis, it shows that the resulting solution after filtration only contains ~6.5% Cr, meaning only about a third of the chromium remained in the solution. Pretty much all the aluminum was separated (~0.25+%) from the precipitate, but the chromium remains a problem. The color of the solution was yellow, meaning the chromium must be in its chromate (CrO₄^-) form.

I did further research and found out chromium is only amphoteric in Cr³⁺ form, so I added FeSO₄ by dissolving iron nails (About 4g) in H₂SO₄, and added it to another sample of the same sludge (About 4g) after leaching it with H₂SO₄ in order to convert hexavalent chromium into trivalent chromium via a redox reaction.

However, upon addition of NaOH until about pH 12, the color of the resulting solution after filtration is still yellow, meaning the chromium is still in its chromate (CrO₄^-) form even after adding FeSO₄, meaning the redox reaction did not occur as planned.

Please advise? Do I need to add more NaOH and make the solution more basic to make more chromium dissolve? I'm not sure what else I can do to separate the chromium, or why the redox reaction didn't work (Does trivalent chromium convert to hexavalent chromium under basic conditions?). Thank you in advance.

EDIT: I have considered using ammonia to form soluble copper and nickel complexes, the problem is I can't use ammonia because my boss doesn't want to utilize it on an industrial level since it will 'create all sorts of other problems' so here I am sadly

[AWK]

Copper and nickel cations form soluble complexes with ammonia. Other cations form insoluble hydroxides.

[JustAnotherPerson95]

Copper and nickel cations form soluble complexes with ammonia. Other cations form insoluble hydroxides.

Ah yes, I have considered that... the problem is I shouldn't be using ammonia because my boss doesn't want to utilize it on an industrial level since it will 'create all sorts of other problems' so here I am

[Enthalpy]

Aqueous electrolysis would deposit Ni and Cu more easily than Cr, Fe and Al. Needs much electricity, but you get the metals reduced.