Over 95% of the world’s zinc is produced from zinc blende (ZnS). Apart from zinc the concentrate contains some 25-30% or more sulphur as well as different amounts of iron, lead and silver and other minerals. Before metallic zinc can be recovered, by using either hydrometallurgical or pyrometallurgical techniques, sulphur in the concentrate must be removed. This is done by roasting or sintering. The concentrate is brought to a temperature of more than 900°C where zinc sulphide (ZnS) converts into the more active zinc oxide (ZnO). At the same time sulphur reacts with oxygen giving out sulphur dioxide which subsequently is converted to sulphuric acid – an important commercial by-product.