When you heat NiAs in a tube furnace at 800-900°C first in air and then in nitrogen, you get a clean nickel powder:

1) in air at 800-900°C: 4NiAs(solid) + 5O2(gas) = 4NiO(solid) + 2As2O3(gas)

2) in nitrogen at 800-900°C: 2NiO(solid) = 2Ni(solid) + O2(gas)

The poisonous As2O3 will sublimated at the cold end of the tube. Beware!

Thank you so much for your valuable guidance . is the residence time in the furnace media important . and if it is , how long should it be ?

Time of the reactions is depends on the amount of material, the more the longer. For 50-100 grams should be enough a half an hour per gas medium.

As2O3 is extremely toxic, the tube furnace should be under a suction. For larger amounts of powder you need a special, water cooled labyrinth separator for As2O3 vapors.