Zinc refining is the process of purifying zinc to special high grade (SHG) zinc, which is at least 99.995% pure. This process is not usually required when smelting of zinc is done through electrolysis processes, but is needed when zinc is produced by pyrometallurgical processes, because it is only 98.5% pure. Zinc refining is the process of purifying zinc to special high grade (SHG) zinc, which is at least 99.995% pure. This process is not usually required when smelting of zinc is done through electrolysis processes, but is needed when zinc is produced by pyrometallurgical processes, because it is only 98.5% pure.