Original: Any impurities remain solid while the nickel carbonyl gas passes into a large chamber at high temperatures in which tens of thousands of nickel spheres, called pellets, are constantly stirred. The nickel carbonyl decomposes, depositing pure nickel onto the nickel spheres.
Simplified: The nickel carbonyl is blown into nickel pellets, where it coats the pellets with pure nickel and puts out carbon monoxide.