Dedalus

Re: Competing Equilibria

Ni 2+(H2O) 6 + 6 NH3 <-> Ni2+ (NH3) 6 + 6 H2O

vs:

Ni 2+ + S2- <-> NiS

The coordination number of the Ni may be less, owing to steric effects, Jahn-Teller effect, etc.

Both this element, and copper form very strong ammonia complexes. The pH affects, largely, this equilibrium:

NH4+ <-> H+ + NH3

The higher the pH, the higher the free ammonia concentration. And, the more sulfide, DTC, TMT; what have you, will be required to drop the Cu and/or Ni. And, inevitably, the more coagulants and other sludge generating crapola you will have to add to get a settlable solid.

Frequently, rather than adding more precipitant, one should finesse the precipitation pH, not neglecting that many of these precipitants (esp. Na2S) are significantly alkaline and affect the pH themselves.

Attention to this can save you lots of $, by avoiding the generation of unnecessary sludge.

Dedalus Environmental - The On-Site Treatment Specialists