Abstract: An agglomeration process for metallurgical by-products and waste products is described, utilizing the sulphate material present in the metallurgical waste and by-products, which involves reacting the sulphate with water and optionally with an added alkaline earth metal compound. Sulphuric acid may also be added to the particles to be agglomerated. The obtained mixture is extruded or cast, and allowed to harden before being used in recycling to an extractive process. The agglomeration mechanism involves one or more of, hydration of a water soluble sulphate, precipitation of a water insoluble alkaline earth metal sulphate and hydration of a water insoluble sulphate.