From Wikipedia HPAL process High pressure acid leach processing...

From Wikipedia

HPAL process

High pressure acid leach processing is employed for two types of nickel laterite ores.

1) Ores with a limonitic character such as the deposits of the Moa district in Cuba and southeast New Caledonia at Goro where nickel is bound in goethite and asbolite.
2) Ores of a predominantly nontronitic character, such as many deposits in Western Australia, where nickel is bound within clay or secondary silicate substrates in the ores. The nickel (+/- cobalt) metal is liberated from such minerals only at low pH and high temperatures, generally in excess of 250 °C.

The advantages of HPAL plants are that they are not as selective toward the type of ore minerals, grades and nature of mineralisation. The disadvantage is the energy required to heat the ore material and acid, and the wear and tear hot acid causes upon plant and equipment. Higher energy costs demand higher ore grades.

Heap (Atmospheric) Leach

Heap leach treatment of nickel laterites is primarily applicable to clay-poor oxide-rich ore types where clay contents are low enough to allow percolation of acid through the heap. Generally, this route of production is much cheaper - up to half the cost of production - due to the lack of need to heat and pressurise the ore and acid.

Ore is ground, agglomerated, and perhaps mixed with clay-poor rock, to prevent compaction of the clay-like materials and so maintain permeability. The ore is stacked on impermeable plastic membranes and acid is percolated over the heap, generally for 3 to 4 months, at which stage 60% to 70% of the nickel-cobalt content is liberated into acid solution, which is then neutralised with limestone and a nickel-cobalt hydroxide intermediate product is generated, generally then sent to a smelter for refining.
The advantage of heap leach treatment of nickeliferous laterite ores is that the plant and mine infrastructure are much cheaper - up to 25% of the cost of a HPAL plant - and less risky from a technological point of view. However, they are somewhat limited in the types of ore which can be treated.