It's dependent on the type of mineralisation (generally the...

It's dependent on the type of mineralisation (generally the amount of serpentine/goethite).

Basically you want to be able to dissolve the nickel, cobalt etc out without also leaching out other low-value metals that end up just wasting reagents (and therefore money).

By using different acids or acid mixes, as well as changing the amount of dilution (e.g. adding more or less water when making the acid) and altering the time/temperature will all change the effectiveness.

Sulphuric is the main acid used, mainly due to cost. I think Hydrochloric is more expensive (I think 50% or so, though I could be making this up and probably more depends on transport costs than the actual cost). There's other acids like Nitric and Citric that can be used, as well as proprietary blends (mostly dialkylphosphinic acids etc).

Tuning the actual extraction depends on the exact mineral composition, educated guesswork and testing/optimisation.

The waters get even more muddy with acid recovery etc.

Edit: Usual disclosures - DYOR, I am not quite a process engineer, don't try this at home.