Leachability of heavy metals and arsenic from slags of metal extraction industry at Angleur (eastern Belgium)

At Angleur (eastern Belgium), a 53,000 m3 dump site, related to former Zn extraction, contains high concentrations of heavy metals and arsenic associated with minerals such as arsenopyrite (FeAsS), franklinite (ZnFe2O4) and willemite (Zn2SiO4). Eight representative samples were investigated by means of a modified availability test, which pointed towards a considerable availability of Cd, Pb and Zn. In addition a detailed analysis of leaching behaviour as a function of pH was performed by means of a pHstat leaching test. The results of this test were supported by modelling predictions with MINTEQA2. pHstat titration results also allowed determination of the acid neutralising capacity, making it possible to predict heavy metal release upon acidification, assuming a worst-case scenario. On a short- to medium-long term (± 100 years) a release of 900 mg/kg Zn has to be expected upon assumed natural acidification. However, a leaching test performed at the natural pH (i.e. pH 8 for the sample studied) indicates that heavy metal release is very limited and far below the European limit values for slag material on dumps category 1. Therefore, despite the elevated total heavy metal and As concentrations on the site, leaching is relatively low under present environmental conditions. Moreover, the slag material has a rather elevated pH (6.5–8) and no acid mine drainage is produced. Glass phases seem very important in controlling heavy metal mobility since potential reactive minerals that are locked up in a silicate matrix are protected against leaching.