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%).

Because of their multi-element chemical composition and especially because of the presence of trace metal elements (TME) at relatively high levels, sludge and slag from foundries are classified as hazardous waste. For this reason, they must undergo all the rigorous management imposed on such types of waste. However, those in charge of the Kinshasa scrap foundries remain deaf to this.Indeed, these scraps from the scrap metal foundries of Kinshasa are spread in plots, streets and avenues of the city of Kinshasa to be used as backfill without prior treatment to the detriment of a suffering population subject to serious environmental problems (potholes, erosion heads, floods, etc.).After chemical analysis of these wastes, it turns out that they contain Na2O, CaO, Cr2O3, MnO, Fe2O3, NiO, CuO, La2O3, Eu2O3, Yb2O3, OsO4, Br, SiO2, K2O, TiO2, V2O5, ZnO, Ti2O3, Au, Sc2O3, SrO, ZrO2, Nb2O5, RuO2, BaO, Re2O7, Al2O3, MgO, Rb2O, HgO [1], while the chemical analysis of the soils backfilled by these slags and sludges of the foundries reveals that they contain all these components except Na2O, Br, Ti2O3, Nb2O5, BaO and Rb2O. Also, some typical components such as AsO3, As2O4, PbO and P2O5 which were not measured in the slag and sludge of the smelters were measured in the soils backfilled by the said slag and sludge.Furthermore, the comparison of the chemical composition of the soils backfilled by SBFs with that of the soils not affected by the backfill shows that P2O5, Al2O3, MgO, OsO4, La2O3, Yb2O3 and SO3, components of the backfilled soils, were not measured in the non-filled soils (blank sample).From a semi-quantitative point of view, it has just been demonstrated that almost all components of the soils backfilled with slag and sludge from smelters have relatively higher concentrations than their counterparts in the unfilled soils, with the exception of SiO2, V2O5 and Sc2O3.

The iron and steel industries generate large quantities of solid waste which is not always easy to manage because of the heavy metals it contains. In most cases, they are used in concrete or bitumen but sometimes as backfill; however, this always requires rigorous control followed by prior treatment. In general, the installation of a foundry requires the creation of a place to store solid waste (slag heap). The scrap metal foundries in Kinshasa do not have appropriate structures for the evacuation or conservation of their waste. They are satisfied with the environmental failure found in the surroundings to liquidate these dangerous wastes in full view of the competent authorities.During our investigations carried out in a sector which contains the aforementioned foundries and whose measured surface is 523272 ha, seventy-six sites on which were spread the slags and muds of the recycling foundries of the scrap were located. The calculation made on the estimates of the weight of these scraps indicates that on average a site contains 592 tons of slag and sludge of the said foundries. These are spread over an average area of 923 m2 and occupy an average volume of 355 m3. In sum, over an average period of 10 years, nearly 45,000 tons of slag and sludge from the scrap metal foundries have been spread on the streets and avenues of our study area over a total surface area of 7,0152 m2 and a relative volume of 26,971 m3.After analysis of the field data, it appears that Block 4 contains a large quantity of slag and sludge from the foundries but also the largest surface area and volume. This is related to the socio-environmental configuration of the area where not only are there several potholes but also the area has no drainage channels for rainwater (hence the recurrent and persistent flooding).In the principal component analysis (PCA), the seventy-six sites are divided into three different clusters, the first of which has only one site, the second has six sites, and the last has the majority of sites, namely sixty-nine. The sites in classes one and two are distinguished by variables (weight, area and volume) that are higher than their respective averages, while those in class three are distinguished by data that are lower than their averages.The evaluation of the daily production of slag and sludge of the said foundries indicates a relative volume of 16 tons. This gives a projected total of 60226 tons of slag and sludge since the installation of these foundries in the city of Kinshasa. The calculation of the difference proves that our study area contains at least 75% of this waste, without counting those contained in third party plots. Furthermore, our investigations attest that a part of the slag and sludge from the foundry that was not evaluated was dumped in the districts bordering our study area, which have similar topographic and environmental characteristics.