Primary aluminum is produced by the Hall-Heroult process, the electrolysis of alumina dissolved in a molten cryolite-based electrolyte. Electric current is used to separate the alumina into aluminum and oxygen. Alumina itself is produced from bauxite ore feedstock in a thermal digestion process. Scrap pretreating and melting to produce secondary aluminum takes place in fuel-fired (or occasionally electric) furnaces. Forming processes use mainly electricity to drive casting machines, rolling mills, and other forming and finishing equipment.