Valuable high purity lead compound and eventually metallic lead is obtained from minerals as well as from electrode paste or slime recovered from dismissed lead batteries by pyrometallurgy according either to a very high temperature (1500-1700° C.) process with attendant generation of large quantities toxic fumes to be abated, or to a high temperature (1000-1100° C.) process, as often practiced in case the starting material is recovered electrode paste, that implies a significant by-production of toxic slags to be disposed of as dangerous substances in special dumps.
An attempt to improve these situations has been pursued through a carbonation technique of the raw (impure) electrode paste in plants for producing secondary lead.
This technique gives substantial advantages in terms of reduced impact on the environment but poor economical results and therefore has not encountered diffused acceptance in the industry. Many have endeavoured to improve the performance of this technique as testified by the numerous patent publications such as: U.S. Pat. No. 5,827,347, also published as WO9966105; U.S. Pat. No. 5,840,262 also published as WO9858878; the European Patents No. 1619261, No. 1656463; the Italian Patent No. 01239001; the U.S. Pat. No. 4,336,236 and No. 1,738,081; the BP patents No. 239,257 and No. 272,053; the Belgian Patents No. 273,660 and No. 691,028 as well as the project “Cleanlead Gypsum” partially financiated by the European Union.
Lead is extracted from a lead mineral such as galena, through a high temperature (over 1500° C.) pyro-metallurgical process that produces impure lead. All sulphur present in the starting material is converted to SO2 such that generally to the pyro-metallurgical plant is associated a plant for producing sulphuric acid in order to avoid inertization of SO2 in the form of CaSO4, which has an extremely low commercial value.