The present PhD thesis investigates the quality upgrade of zinc oxides obtained from steel mill dust recycling processes. The driving force is the possibility to get a product with added value by avoiding the usage as substitute for primary zinc concentrates and alternatively utilize it in the high purity zinc oxide market or alternative areas, like the zinc chemicals market in case of zinc sulphate. Therefore, specified metallurgical processes are required to ameliorate the quality of such zinc oxides. Due to the dominance of the waelz process in the recycling of steel mill dusts in Europe, especially the waelz oxide was investigated. Based on a detailed characterization and evaluation of physical and chemical properties of the constituent parts as well as the requirements of the target markets, a hydrometallurgical concept for the production of zinc sulphate and a pyrometallurgical concept for the selective vaporization of impurities of waelz oxide were studied. Thermochemical calculations showed next to detailed experimental work that within the first process concept, the selective vaporization, interaction reactions of halides with the main compounds occur and that those reactions can be utilized to enhance the upgrade process. This can lead to lower required treatment temperatures, shorter retention times or higher vaporization yields of disturbing compounds. The second upgrade process was a hydrometallurgical concept for zinc sulphate production. Based on an adaption of hydrometallurgical process steps such as cementation, precipitation and selective leaching regarding the input material, it was shown that impurities could be effectively separated. An upgraded product, clean zinc sulphate, was produced and an added value was realized. With this, the possibility of breaking into new seminal markets with a possible second product line for waelz kiln operators can be realized and additionally offers a field of new customers.