The Lubumbashi Slag HillThe Lubumbashi slag was created from 1924 to 1992 by the accumulation of scorias originating from the metallurgical activities of the UNION MINIÈRE DU HAUT KATANGA (UMHK), then that of GÉCAMINES.

The structure of the slag hill bears witness to three different phases of exploitation. Only the third is concerned by the GTL-STL project.

The inner part of the slag consists in metallurgical waste and minerals originating from the mines around Lubumbashi, presumably the mines of Étoile and Ruashi.

Covering the inner part, the second and main layer of slag consists in waste from the exploitation of minerals originating from the Kipushi mine.

Lastly, the southern part of the slag, the most recent and the only part to be exploited by GTL-STL, is constituted of waste from mineral exploitation essentially coming from Kolwezi. This is estimated at 4.5 million tonnes of scoria, almost a third of the slag hill. The average cobalt content is about 2.1%. However, the metal present in the greatest proportion is zinc.