Circular Economy
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Around 2.5 million tons of municipal sewage sludge (dry matter) are produced in Ger-many every year. The disposal or recycling of this mass often confronts plant operators with considerable problems. In addition to organic substances, sewage sludges also contain important inorganic nutrients that can be recycled. Due to the finite nature of phosphorus as a resource and in favor of natural material cycles, it makes sense to utilize the phosphorus bound in sewage sludge. Therefore, Phosphorus recycling from sewage sludge will be obligatory in Germany from 2029. However, the pollutants pre-sent in sewage sludge, such as heavy metals and organic trace pollutants, are prob-lematic. In many cases, the sewage sludge is incinerated, removing organic compo-nents, and leaving a mineral residue. Various processes exist for the reprocessing of sewage sludge ashes. Reliable analytics and process monitoring are required for all of them.
In this context, the first and most important task of BAM regarding new materials or processes is safety in technology and chemistry through validated and correct analyt-ics. Since there are no reference materials for sewage sludge ashes so far, it is not possible to make any statements about the extent to which the measured values of the comparative measuring methods are correct at all. Therefore, suitable, and representa-tive reference materials are indispensable for the validation of the measurement results and the quality assurance during the entire recycling process. Great care is required to select and prepare these reference materials. The reference materials must corre-spond in matrix to the samples that are to be investigated in the industrial process. An iron-rich and an aluminum-rich sewage sludge ash from mono-incineration plants were selected for this purpose. 10 kg of each sewage sludge ash were homogenized, char-acterized, and prepared. The main, minor and trace element compositions of the sew-age sludge ashes are determined by different measuring methods. The candidate ref-erence materials will be available as BAM-U200 und BAM-U201.
The leaching behavior of scandium (Sc) from bauxite residues can differ significantly when residues of different geological backgrounds are compared. The mineralogy of the source rock and the physicochemical environment during bauxitization affect the association of Sc in the bauxite i.e., how Sc is distributed amongst different mineral phases and whether it is incorporated in and/or adsorbed onto those phases. The Sc association in the bauxite is in turn crucial for the resulting Sc association in the bauxite residue. In this study systematic leaching experiments were performed on three different bauxite residues using a statistical design of experiments approach. The three bauxite residues compared originated from processing of lateritic and karstic bauxites from
Germany, Hungary, and Russia. The recovery of Sc and Fe was determined by ICP-OES measurements. Mineralogical changes were analyzed by X-ray-diffraction and subsequent Rietveld refinement. The effects of various parameters including temperature, acid type, acid concentration, liquid-to-solid ratio and residence time were studied. A response surface model was calculated for the selected case of citric acid leaching of Hungarian bauxite residue. The investigations showed that the type of bauxite residue has a strong influence. The easily leachable fraction of Sc can vary considerably between the types, reaching ~20–25% in German Bauxite residue and ~50% in Russian bauxite residue. Mineralogical investigations revealed that a major part of this fraction was released from secondary phases such as cancrinite and katoite formed during Bayer processing of the bauxite.
The effect of temperature on Sc and Fe recovery is strong especially when citric acid is used. Based on the exponential relationship between temperature and Fe-recovery it was found to be particularly important for the selectivity of Sc over Fe. Optimization of the model for a maximum Sc recovery combined with a minimum Fe
recovery yielded results of ~28% Sc recovery at <2% Fe recovery at a temperature of 60 ◦C, a citric acid normality of 1.8, and a liquid-to-solid ratio of 16 ml/g. Our study has shown that detailed knowledge about the Sc association and distribution in bauxite and bauxite residue is key to an efficient and selective leaching of Sc
from bauxite residues.