Chemical characterization of slag and sludge from foundries that recycle scrap metal in Kinshasa-DRC: Relative quality and quantity of pollutants from the substratum backfilled with these materials

For nearly a decade and a half, some foundries that recycle scrap metal have been established in Kinshasa, the capital of the Democratic Republic of Congo. These foundries produce annually, as do other foundries, thousands of tons of solid waste, the mismanagement of which was demonstrated in our previous article in the same press. The waste from the foundries being counted among the dangerous waste, it immediately seemed useful to us to determine their chemical composition.After chemical analyses by X-ray fluorescence spectrophotometry (XRF) on 15 samples of slag and 15 samples of sludge from three of the said foundries, the following results were obtained (i) qualitatively, the sludge of the foundries is composed of Na2O, CaO, Cr2O3, MnO, Fe2O3, NiO, CuO, La2O3, Eu2O3, Yb2O3, OsO4, Br, SiO2, K2O, TiO2, V2O5 and ZnO while the slag contains in addition to these last components Ti2O3, Sc2O3, SrO, ZrO2, Nb2O5, RuO2, BaO, Re2O7, Al2O3, Au, MgO, Rb2O and HgO, (ii) On the semi-quantitative level, the chemical analyses indicate that Fe2O3 is the most preponderant component in the sludge with an average mass concentration evaluated at more than 85%, followed by Na2O [6.33%], SiO2 [5.3%], MnO [1.5%]. Eu2O3, Cr2O3 and CuO have concentrations in the order of the tenth (10-1), while the majority of the remaining components have concentrations in the order of the hundredth (10-2), except Re2O7, ZrO2, SrO and V2O5 whose concentrations are small in the order of the thousandth (10-3). On the other hand, the main component of the slag is SiO2 concentrated at 36% of the average total weight, followed by Fe2O3 [23%], MnO [15.5%], Al2O3 [10%], Cr2O3 [1.5%], Na2O [1.1%] which are the major components. TiO2, BaO, ZnO, MgO, SrO, K2O, V2O5, RuO2 have concentrations on the order of tenths ranging from 0.8 ≥ x ≥ 0.112, ZrO2, Eu2O3, Nb2O5, Re2O7 have concentrations on the order of hundredths ranging from 0, 047 ≥ x ≥ 0.02 and NiO, Au, Sc2O3, Ti2O3 and Br have relative concentrations on the order of thousandths ranging from 0.00333% ≥ x ≥ 0.001%.The comparison of the different compositions of the sludge and slag indicates that the slag from the FAMECO smelter, not only has several components related to the other two smelters, but also has higher or lower relative concentrations than the other two. Finally, the pooling or mixing of sludge and slag at the time of disposal indicates that the contribution of pollutants from the slag is far more abundant (78%) than from the sludge (22%).