sorry wrong here, saprolites (high Mg) are more amenable to...

sorry wrong here, saprolites (high Mg) are more amenable to processing via pyrometallurgy, not HPAL
limonite (low Mg) -> HPAL
DNi process is one exception, using nitric acid hydrometallurgy

"The high magnesium contentin saprolite laterites results in highsulfuric acid consumption – up to500 kilograms per tonne of ore.The elegance of Direct Nickel’s DNiprocess is the use of nitric acid – it ismuch more aggressive than sulfuric, soit eliminates long leach times or hightemperatures and pressures. And it isalmost wholly recyclable – net nitricacid consumption rates are as low as40 kilograms per tonne of ore (albeit atthree times the price of sulfuric)."
http://dcstechnical.com.au/wp-content/uploads/2013/10/Chemistry-in-Australia-Nov-2013.pdf