Open Access

Editorial zur Topical Collection ANAKON 2025

Introduction We present the first results of a pilot-scale study on the effect of bulk chemistry, oxidation state and cooling practice on EAF-slag mineralogy, microstructure and leaching behaviour. Methods 24 batches of synthetic EAF-slag were produced (ca. 80-100 kg per batch) by remelting industrial slag in a pilot electric-arc furnace and trimming the chemistry with additives to a suite of target compositions. Two different cooling practices were used for the main batches: 1) cooling under air, and 2) granulation with water. Smaller (ca. 500 g) spoon-samples were taken prior to tapping, with a cooling rate intermediate between the two. The produced batches span a wide range of chemical composition regarding total FeOx content (ca. 20 - 40 wt%), Fe3+/Fe(total) ratio (0.15 - 0.80), CaO/SiO2 (B2 = 1.0 - 2.2), Al2O3/SiO2 (A/S = 0.2 - 0.7) and MgO/SiO2 (M/S = 0.4-1.1). The targeted compositions simulate a) a range of scenarios of operational tap-slag composition, b) high-temperature modification of some of these by addition of SiO2 or Al2O3 to lower B2 or raise A/S after tapping, respectively, and c) variable oxidation state prior to solidification. For the scenarios (a) representing operational tap-slag compositions, the B2, A/S and total FeOx were primary experimental variables, while MgO content was adjusted for each to satisfy a consistently applied saturation criteria in solid Mg-wüstite at operational temperatures, resulting in a range from 7.6 to 16.3 wt%. The vanadium and chromium contents of the slags in those scenarios were targeted at around 1 wt% each. For the scenarios (b), the additions dilute all the other components in the slag. The mineralogy and amorphous content of the slags was analysed with the XRD-Rietveld method, using added metallic silicon standard to quantify amorphous content. The phase chemistry and microstructure was studied with SEM-EDS analysis. Results The air-cooled slags with highest B2 have C2S ± bredigite as the dominant silicate phases, while those with lower B2 have Ca-rich olivine ± merwinite as the dominant silicate phases with minor melilite. The total FeOx content is strongly correlated with the total fraction of crystalline Fe-Mg-Mn(-Cr-Al)-oxides in monoxide (Mg-wüstite) or spinel form, while the spinel/monoxide ratio correlates strongly with the Fe3+/Fe(tot) ratio. Differences in Al2O3/MgO also influence the spinel/monoxide ratio and strongly influence the C2S/bredigite and olivine/merwinite/melilite ratios at high and low B2, respectively. Water granulation of the same range of slag compositions yields products with widely variable amorphous glass content, ranging from < 10 wt% through to 73 wt%. The amorphous content and crystalline phase occurrence is strongly correlated with the bulk chemistry of the slags and, moreover, has a clear link with the predicted phase assemblage from thermodynamic equilibrium calculations (Factsage). The leaching tendency of vanadium is strongly correlated with the mineralogy of the samples, and is well described by a simple multi-linear regression comprising only three variables (mass fractions of key mineralogical phases). Conclusions Suppressing the formation of C2S in favour of more bredigite diminishes the V-leaching tendency, as does suppressing Ca-rich olivine in favour of merwinite. The amorphous content has a strong negative correlation with V-leaching. V- and Si-leaching are positively correlated for the studied slags. This suggests a mechanistic control by the relative ease of dissolution of the V-bearing silicate phases together with their V-content, where the amorphous glass phase corresponds with lowest Si and V leaching tendency, and C2S the highest.

In Deutschland wird Klärschlamm häufig in Monoverbrennungsanlagen thermisch verwertet. Wir haben von den meisten Anlagen Aschen eingesammelt und diese auf deren chemische Zusammensetzung analysiert. Abhängig von der Zusammensetzung können die Aschen in unterschiedliche Kategorien eingeteilt werden, mit welchem P-Rückgewinnungsverfahren diese aufbereitet werden könnten. Sowohl aus diesen Klärschlammaschen als auch aus dem Abwasser und den Klärschlamm können verschiedene Rezyklate hergestellt werden. Dies können Struvite, thermochemische Produkte, Calciumphosphate und Phosphorsäure sein.

A thermochemical process for phosphorus recovery has been integrated into the sewage sludge co-incineration plant operated by Emter GmbH in Altenstadt. During incineration, the sewage sludge reacts with sodium additives, producing a highly effective inorganic phosphate fertilizer. The plant produces up to 15,000 metric tons of the recycled material per year. The high fertilizing effectiveness of the recycled material has been successfully demonstrated in pot and field trials.

In Deutschland wird Klärschlamm häufig in Monoverbrennungsanlagen thermisch verwertet. Wir haben von den meisten Anlagen Asche eingesammelt und diese auf deren chemische Zusammensetzung analysiert. Abhängig von der Zusammensetzung können die Aschen in unterschiedliche Kategorien eingeteilt werden, mit welchem P-Rückgewinnungsverfahren diese aufbereitet werden könnten