An additive processing apparatus includes a chamber and an irradiation device that together are operable to additively fabricate an article from a powder layer-by-layer in a work space in the chamber. A gas recirculation loop is connected at a first end thereof to an outlet of the chamber and at a second end thereof to an inlet of the chamber. The gas recirculation loop includes at least one purification device that is configured to remove impurities from a cover gas and generate a clean cover gas.