Abstract: A metal-refining process and a corresponding system characterized by being adapted to minimize the health hazard by appropriate purification of the gases or vapor products resulting from acid action on a metal compound. This metal refining features the use of vacuum at a downstream point to regulate the admixing of air with the vapor products in the effluent and to draw activating air in the mixture of acid and metal compound at the upstream point. This refining system includes gas outlet manifolds constructed and arranged to enhance the purification of the vapor products by comminuting the vapor products into small bubbles upwardly through a water tank. Each manifold consists of a casing having a plurality of small gas outlet apertures each communicating with a small tube which extends within the casing and has a bevelled end for self-cleaning of the tube. These tubes prevent clogging of the apertures by solid particles in the gas flowing through the casing.