A process for recovering nickel from a solid feed material containing nickel in the form of millerite is disclosed. The process includes treating a slurry of the solid feed material and a process solution so that all or at least part of the nickel in the millerite forms an acid leachable solid form. The process also includes pressure acid leaching the treated slurry and leaching nickel in solids in the slurry into solution. Finally, the process includes recovering nickel from solution.